Formation of thin film resistors

ABSTRACT

A method is provided for forming a patterned layer of resistive material in electrical contact with a layer of electrically conducting material. A three-layer structure is formed which comprises a metal conductive layer, an intermediate layer formed of material which is degradable by a chemical etchant, and a layer of resistive material of sufficient porosity such that the chemical etchant for said intermediate layer may seep through the resistive material and chemically degrade said intermediate layer so that the resistive material may be ablated from said conductive layer wherever the intermediate layer is chemically degraded. A patterned photoresist layer is formed on the resistive material layer. The resistive material layer is exposed to the chemical etchant for said intermediate layer so that the etchant seeps through the porous resistive material layer and degrades the intermediate layer. Then, portions of the resistive material layer are ablated away wherever the intermediate layer has been degraded.

This is a Continuation-In-Part of copending application Ser. No.09/069,679, filed on Apr. 29, 1998, U.S. Pat. No. 6,210,952.

The present invention is directed to the formation of thin layerresistors, preferably for printed circuitry, such thin layers beingcapable of being embedded within a printed circuit board. In particular,the invention is directed to forming thin layer resistors from thinlayers of resistive material which may be deposited by combustionchemical vapor deposition.

BACKGROUND OF THE INVENTION

Combustion chemical vapor deposition (“CCVD”), a recently invented CVDtechnique, allows for open atmosphere deposition of thin films. The CCVDprocess offers several advantages over other thin-film technologies,including traditional CVD. The key advantage of CCVD is its ability todeposit films in the open atmosphere without any costly furnace, vacuum,or reaction chamber. As a result, the initial system capitalizationrequirement can be reduced up to 90% compared to a vacuum based system.Instead of a specialized environment, which is required by othertechnologies, a combustion flame provides the necessary environment forthe deposition of elemental constituents from solution, vapor, or gassources. The precursors are generally dissolved in a solvent that alsoacts as the combustible fuel. Depositions can be performed atatmospheric pressure and temperature within an exhaust hood, outdoors,or within a chamber for control of the surrounding gasses or pressure.

Because CCVD generally uses solutions, a significant advantage of thistechnology is that it allows rapid and simple changes in dopants andstoichiometries which eases deposition of complex films. The CCVDtechnique generally uses inexpensive, soluble precursors. In addition,precursor vapor pressures do not play a role in CCVD because thedissolution process provides the energy for the creation of thenecessary ionic constituents. By adjusting solution concentrations andconstituents, a wide range of stoichiometries can be deposited quicklyand easily. Additionally, the CCVD process allows both chemicalcomposition and physical structure of the deposited film to be tailoredto the requirements of the specific application.

Unlike conventional CVD, the CCVD process is not confined to anexpensive, inflexible, low-pressure reaction chamber. Therefore, thedeposition flame, or bank of flames, can be moved across the substrateto easily coat large and/or complex surface areas. Because the CCVDprocess is not limited to specialized environments, the user cancontinuously feed materials into the coating area without disruption,thereby permitting batch processing. Moreover, the user can limitdeposition to specific areas of a substrate by simply controlling thedwell time of the flame(s) on those areas. Finally, the CCVD technologygenerally uses halogen-free chemical precursors having significantlyreduced negative environmental impact.

Numerous materials have been deposited via CCVD technology with thecombustion of a premixed precursor solution as the sole heat source.This inexpensive and flexible film deposition technique permits broaduse of thin film technology. The CCVD process has much of the sameflexibility as thermal spraying, yet creates quality, conformal filmslike those associated with conventional CVD. With CCVD processing, adesired phase can be deposited in a few days and at relatively low cost.

A preferred embodiment of the CCVD process is described in detail inU.S. application Ser. No. 08/691,853 filed Aug. 2, 1996, the teachingsof which are incorporated herein by reference. In accordance with thatapplication, a CCVD produces vapor formed films, powders and nanophasecoatings from near-supercritical liquids and supercritical fluids.Preferably, a liquid or liquid-like solution fluid containing chemicalprecursor(s) is formed. The solution fluid is regulated to near or abovethe critical pressure and is then heated to near the supercriticaltemperature just prior to being released through a restriction or nozzlewhich results in a gas entrained very finely atomized or vaporizedsolution fluid. The solution fluid vapor is combusted to form a flame oris entered into a flame or electric torch plasma, and the precursor(s)react to the desired phase in the flame or plasma or on the substratesurface. Due to the high temperature of the plasma much of the precursorwill react prior to the substrate surface. A substrate is positionednear or in the flame or electric plasma, and a coating is deposited.Alternatively, the material formed can be collected as a nanophasepowder.

Very fine atomization, nebulization, vaporization or gasification isachieved using solution fluids near or above the critical pressure andnear the critical temperature. The dissolved chemical precursor(s) neednot have high vapor pressure, but high vapor pressure precursors canwork well or better than lower vapor pressure precursors. By heating thesolution fluid just prior to or at the end of the nozzle or restrictiontube (atomizing device), the available time for precursor chemicalreaction or dissolution prior to atomization is minimized. This methodcan be used to deposit coatings from various metalorganics and inorganicprecursors. The fluid solution solvent can be selected from any liquidor supercritical fluid in which the precursor(s) can form a solution.The liquid or fluid solvent by itself can consist of a mixture ofdifferent compounds.

A reduction in the supercritical temperature of the reagent containingfluid produces superior coatings. Many of these fluids are not stable asliquids at STP, and must be combined in a pressure cylinder or at a lowtemperature. To ease the formation of a liquid or fluid solution whichcan only exist at pressures greater than ambient, the chemicalprecursor(s) are optionally first dissolved in primary solvent that isstable at ambient pressure. This solution is placed in a pressurecapable container, and then the secondary (or main) liquid or fluid(into which the primary solution is miscible) is added. The main liquidor fluid has a lower supercritical temperature, and results in alowering of the maximum temperature needed for the desired degree ofnebulization. By forming a high concentration primary solution, much ofthe resultant lower concentration solution is composed of secondary andpossible additional solution compounds. Generally, the higher the ratioof a given compound in a given solution, the more the solutionproperties behave like that compound. These additional liquids andfluids are chosen to aid in the very fine atomization, vaporization orgasification of the chemical precursor(s) containing solution. Choosinga final solution mixture with low supercritical temperature additionallyminimizes the occurrence of chemical precursors reacting inside theatomization apparatus, as well as lowering or eliminating the need toheat the solution at the release area. In some instances the solutionmay be cooled prior to the release area so that solubility and fluidstability are maintained. One skilled in the art of supercritical fluidsolutions could determine various possible solution mixtures withoutundue experimentation. Optionally, a pressure vessel with a glasswindow, or with optical fibers and a monitor, allows visualdetermination of miscibility and solute-solvent compatibility.Conversely, if in-line filters become clogged or precipitant is foundremaining in the main container, an incompatibility under thoseconditions may have occurred.

Another advantage is that release of fluids near or above theirsupercritical point results in a rapid expansion forming a high speedgas-vapor stream. High velocity gas streams effectively reduce the gasdiffusion boundary layer in front of the deposition surface which, inturn, improves film quality and deposition efficiency. When the streamvelocities are above the flame velocity, a pilot light or other ignitionmeans must be used to form a steady state flame. In some instances twoor more pilots may be needed to ensure complete combustion. With theplasma torch, no pilot lights are needed, and high velocities can beeasily achieved by following operational conditions known by one ofordinary skill in the art.

The solute-containing fluid need not be the fuel for the combustion.Noncombustible fluids like water N₂O or CO₂, or difficult to combustfluids like ammonia, can be used to dissolve the precursors or can serveas the secondary solution compound. These are then expanded into a flameor plasma torch which provides the environment for the precursors toreact. The depositions can be performed above, below or at ambientpressure. Plasma torches work well at reduced pressures. Flames can bestable down to 10 torr, and operate well at high pressures. Cool flamesof even less than 500° C. can be formed at lower pressures. While bothcan operate in the open atmosphere, it can be advantageous to practicethe methods of the invention in a reaction chamber under a controlledatmosphere to keep airborne impurities from being entrained into theresulting coating. Many electrical and optical coating applicationsrequire that no such impurities be present in the coating. Theseapplications normally require thin films, but thicker films for thermalbarrier, corrosion and wear applications can also be deposited.

Further bulk material can be grown, including single crystals, byextending the deposition time even further. The faster epitaxialdeposition rates provided by higher deposition temperatures, due tohigher diffusion rates, can be necessary for the deposition of singlecrystal thick films or bulk material.

CCVD is a flame process which utilizes oxygen. While it may be possibleusing CCVD to deposit oxygen-reactive materials with CCVD by depositingin the reducing portions of the flame, a better technique for depositingoxygen reactive materials, such as nickel, is a related processdescribed in U.S. patent application Ser. No. 09/067,975, filed Apr. 29,1998, the teachings of which are incorporated herein by reference.

The invention described in referenced U.S. patent application Ser. No.09/067,975 provides an apparatus and method for chemical vapordeposition wherein the atmosphere in a coating deposition zone isestablished by carefully controlling and shielding the materials fed toform the coating and by causing the gases removed from the depositionzone to pass through a barrier zone wherein they flow away from saiddeposition zone at an average velocity greater than 50 feet per minute,and preferably greater than 100 feet per minute. The rapid gas flowthrough the barrier zone essentially precludes the migration of gasesfrom the ambient atmosphere to the deposition zone where they couldreact with the coating or the materials from which the coating isderived. Careful control of the materials used to form the coating canbe provided by feeding the coating precursors in a fixed proportion in aliquid media. The liquid media is atomized as it is fed to a reactionzone wherein the liquid media is vaporized and the coating precursorsreact to form reacted coating precursors. Alternatively, the coatingprecursor(s) can be fed as a gas, either as itself or as a mixture in acarrier gas. The reacted coating precursors are often composed ofpartially, fully and fractionally reacted components, which can flow asa plasma to the deposition zone. The reacted coating precursors contactand deposit the coating on the surface of the substrate in thedeposition zone. A curtain of flowing inert gases may be provided aroundthe reaction zone to shield the reactive coating materials/plasma inthat zone from contamination with the materials used in the surroundingapparatus or with components of the ambient atmosphere.

The vaporization of the liquid media and reaction of the coatingprecursors in the reaction zone requires an input of energy. Therequired energy can be provided from various sources, such as electricalresistance heating, induction heating, microwave heating, RF heating,hot surface heating and/or mixing with hot inert gas.

Herein, non-combustion process will be referred to as “ControlledAtmosphere Combustion Chemical Vapor Deposition” (CACCVD). Thistechnique provides a relatively controlled rate of energy input,enabling high rates of coating deposition. In some preferred cases, theliquid media and/or a secondary gas used to atomize the liquid media canbe a combustible fuel used in the CACCVD. Particularly important is thecapability of CACCVD to form high quality adherent deposits at or aboutatmospheric pressure, thereby avoiding the need to be conducted inelaborate vacuum or similar isolation housings. For these reasons, inmany cases, CACCVD thin film coatings can be applied in situ, or “in thefield”, where the substrate is located.

Combustion chemical vapor deposition (CCVD) is not suitable for thosecoating applications which require an oxygen free environment. For suchapplications, CACCVD, which employs non-combustion energy sources suchas hot gases, heated tubes, radiant energy, microwave and energizedphotons as with infrared or laser sources are suitable. In theseapplications it is important that all of the liquids and gases used beoxygen-free. The coating precursors can be fed in solution or suspensionin liquids such as ammonia or propane which are suitable for the depositof nitrides or carbides, respectively.

CACCVD processes and apparatus provide control over the deposition zoneatmosphere, thereby enabling the production of sensitive coatings ontemperature sensitive or vacuum sensitive substrates, which substratescan be larger than could otherwise be processed by conventional vacuumchamber deposition techniques.

A further advantage of CACCVD is its ability to coat substrates withoutneeding additional energy supplied to the substrate. Accordingly, thissystem allows substrates to be coated which previously could notwithstand the temperatures to which substrates were subjected by mostprevious systems. For instance, nickel coatings can be provided onpolyimide sheet substrates without causing deformation of the substrate.Previously atmospheric pressure deposition techniques were unable toprovide chemical vapor deposition of metallic nickel because of itsstrong affinity to oxygen, while vacuum processing of polyimide sheetsubstrates was problematical due to its outgassing of water and tendencytoward dimensional instability when subjected to heat and vacuum.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of the apparatus of the invention.

FIG. 2 shows a schematic diagram of an apparatus for the deposition offilms and powders using near supercritical and supercriticalatomization.

FIG. 3 shows a detailed schematic view of the atomizer used in thepresent invention.

FIGS. 4a-4 c show in cross-sectional diagrams, steps of forming a thinlayer resistor in accordance with the present invention; FIG. 4d is aplan view of the thin layer resistor of FIG. 4c.

FIGS. 5a-5 c are similar cross-sectional diagrams, illustrating steps offorming thin layer resistors in accordance with alternate processes ofthe invention.

FIG. 6 is a cross-sectional view of the resistor of FIG. 4c embedded ininsulating material.

FIG. 7 is a schematic view, partially in section, of an apparatus forapplying coatings in accord with the present invention.

FIG. 8 is a close-up perspective view, partially in section, of aportion of the coating head used in the apparatus of FIG. 7.

FIGS. 9a-9 g are cross-sectional views of structures representing aprocess for fabricating a resistor from a free-standing foil coated withan electrically resistive material.

FIGS. 10a, b and c illustrate a process of preparing a resistor patternon a metal foil starting with a three-layer laminate including anelectrically conductive foil layer, an intermediate etchable layer, anda layer of porous resistive material.

SUMMARY OF THE INVENTION

In accordance with the present invention thin layer resistors are formedon a substrate, which resistors may be embedded within a printed circuitboard. On a substrate is formed a thin layer of resistive material.Preferred resistive materials which form the thin layers are homogeneousmixtures of metals, such as platinum, and dielectric materials, such assilica or alumina. Even minor amounts of dielectric material admixedwith a metal significantly increase the resistance of the metal.Preferably, the resistive material is deposited on the substrate bycombustion chemical vapor deposition (CCVD). In the case of zero valencemetals and dielectric material, the homogeneous mixture is achieved byco-deposition of the metal and dielectric material by CCVD. To formdiscrete patches of the resistive material, selected portions of theresistive material layer are etched away. Thus, a layer of resistivematerial may be covered with a patterned resist, e.g., an exposed anddeveloped photoresist, and exposed portions of the underlying layer ofresistive material etched away. Furthermore, the invention provides forthe formation of thin layer, discrete patches of a layer of resistivematerial, and conductive material in electrical contact withspaced-apart locations on the patches of resistive material layer, suchconductive material providing for electrical connection of the resistivematerial patches with electronic circuitry. Such structures ofinsulating material, resistive material, and conductive material may beformed by selective etching procedures.

Certain of the resistive materials which may be deposited by CCVD inaccordance with the invention are porous. Such porosity facilitatesetching by an etchant which attacks the underlying substrate. Selectedportions of a porous resistive material layer are exposed to an etchantwhich seeps through the micropores in the resistive layer and attacksthe underlying substrate material thereby destroying adhesion betweenthe substrate and the resistive material layer. Due to the thinness ofthe resistive material layer, when adhesion is destroyed, the thin layerof resistive material, in those regions exposed to etchant, are brokenup and is carried away in the etchant, e.g., sprayed etchant. Exposureto the etchant is limited to a period of time sufficient to remove(ablate) the resistive material but not long enough to cause significantundercutting of the substrate.

In one embodiment of the invention, the resistive material layer isdeposited on a metal foil, particularly copper foil, which foil is usedto form the conductive circuitry traces in electrical contact with thethin layer resistors of the present invention. Discrete patches ofresistive material are formed by use of photoimaging and ablativeetching. The resistive material layer side of the foil is then embeddedin dielectric material, e.g., prepreg. Then, using photoimaging, thefoil is etched into a circuitry trace pattern. This circuitry tracepattern is likewise embedded in dielectric material.

Because copper and/or copper oxide may interact with the resistivematerial layer as it is being deposited, in one embodiment of theinvention there is deposited a barrier layer on the copper surfacebefore the resistive material layer is deposited. The barrier layer maybe a metal, such as nickel, or a layer of dielectric material, such assilica, which is so thin that it does not disrupt electrical contactbetween the copper foil and the resistive material deposited thereontop.When the resistive material layer is porous, ablative etching may beaccomplished using an etchant which attacks the barrier layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention may be understood more readily by reference to thefollowing detailed description of preferred embodiments of the inventionand the Figures.

It is to be understood that the terminology used herein is for thepurpose of describing particular embodiments only and is not intended tobe limiting. It must be noted that, as used in the specification and theappended claims, the singular forms “a,” “an” and “the” include pluralreferents unless the context clearly dictates otherwise.

Throughout this application, where publications are referenced, thedisclosures of these publications in their entireties are herebyincorporated by reference into this application in order to more fullydescribe the state of the art to which this invention pertains.

The present invention provides a method for coating a substrate with aselected material. The method comprises, at a first selected temperatureand a first selected pressure, dissolving into a suitable carrier tothereby form a transport solution one or more reagents capable ofreacting (where, for a single precursor reagent, the precipitation ofthe reagent from the solution or change in chemical bonds is hereinconsidered a “reaction”) to form the selected material. At some timeprior to the actual deposition, a substrate is positioned in a regionhaving a second selected pressure. The second selected pressure can beambient pressure and is generally above 20 torr. The transport solutionis then pressurized to a third selected pressure above the secondselected pressure using a pressure regulating means. One of skill in theart would recognize that there are many suitable pressure regulatingmeans, including, but not limited to compressors, etc. Next, thepressurized, transport solution is directed to a fluid conduit having aninput end and an opposed output end having a temperature regulatingmeans positioned thereon for regulating the temperature of the solutionat the output end. The output end of the conduit further comprises anoutlet port oriented to direct the fluid in the conduit into the regionand in the direction of the substrate. The outlet port can be of a shapesimilar to a nozzle or restrictor as used in other spraying andatomizing applications. Thereafter, the solution is heated using thetemperature regulating means to a second selected temperature within 50°C. above or below the critical temperature, T_(c), of the solution whilemaintaining the third selected pressure above the second selectedpressure and above the corresponding liquidus or critical pressure,P_(c,) of the solution at the second selected temperature using thepressure regulating means. Then, the pressurized, heated solution isdirected through the outlet port of the conduit into the region toproduce a nebulized solution spray in the direction of the substrate. Asthe solution is directed into the region, one or more selected gases areadmixed into the nebulized solution spray to form a reactable spray and,thereafter, this reactable spray is exposed to an energy source at aselected energization point. The energy source provides sufficientenergy to react the reactable spray (which contains the one or morereagents of the transport solutions) thereby forming the material andcoating the substrate therewith.

In a further embodiment of this method, the energy source comprises aflame source and the selected energization point comprises an ignitionpoint. In an alternate embodiment. The energy source comprises a plasmatorch.

In a further embodiment of the method, the second selected pressure ofthe region is ambient pressure.

In yet another embodiment, the nebulized solution spray is a vapor or anaerosol having a maximum droplet size of less than 2 μm.

In a further embodiment, the second selected pressure of the region isreduced to produce a combustion flame having a temperature of less than1000° C.

In yet another embodiment, the carrier is propane and the transportsolution comprises at least 50% by volume propane. In a furtherembodiment, the transport solution further includes butanol, methanol,isopropanol, toluene, or a combination thereof In yet anotherembodiment, the carrier is selected such that the transport solution issubstantially precipitate free at standard temperature and pressure fora period of time sufficient to carry out the method.

In an alternate embodiment of the method, a pressurized container isused and before, during or after the pressuring step, a standardtemperature and pressure gas is also contacted with the transportsolution at a selected pressure sufficient to form a liquid orsupercritical fluid (depending upon the temperature). In a preferredembodiment, the transport solution containing the standard temperatureand pressure gas is substantially precipitate free at the selectedpressure for a period of time sufficient to carry out the method. In yetanother embodiment, the reagent concentration of the transport solutionis between 0.0005 M and 0.05 M.

In a further embodiment, the outlet end of the conduit further comprisesa fluid introduction port and, prior to directing the pressurized,heated solution through the outlet port of the conduit, fluid is addedto the pressurized, heated solution through the fluid introduction port.Such introduction forms a combined solution having a reducedsupercritical temperature.

In yet another embodiment, each of the one or more reagents has a vaporpressure of no less than about 25% of the vapor pressure of the carrier.

In a further embodiment, the outlet end of the conduit comprises tubinghaving an internal diameter of 2 to 1000 μm, more preferably 10 to 250μm. In a more preferable embodiment, the outlet end of the conduitcomprises tubing having an internal diameter of 25 to 125 μm. In yet afurther preferable embodiment, the outlet end of the conduit comprisestubing having an internal diameter of 50 to 100 μm.

In another embodiment, the temperature regulating means comprises meansfor resistively heating the conduit by applying thereto an electriccurrent of a selected voltage from an electric current source. In apreferred embodiment, the voltage is less than 115 Volts. In yet anotherpreferred embodiment, the means for resistively heating the conduitcomprises a contact positioned within 4 mm of the outlet port.

Moreover, the present invention also provides the above method whereinthe carrier and one or more reagents are selected such that the secondselected temperature is ambient temperature.

The above method may be practiced wherein the material that coats thesubstrate comprises a metal, a metal or metalloid oxide, or a mixture ofa metal with a metal or metalloid oxide.

In a further embodiment, the reactable spray comprises a combustiblespray having a combustible spray velocity and wherein the combustiblespray velocity is greater than the flame speed of the flame source atthe ignition point and further comprising one or more ignitionassistance means for igniting the combustible spray. In a preferredembodiment, each of the one or more ignition assistance means comprisesa pilot light. In yet another embodiment, the combustible spray velocityis greater than mach one.

In a further embodiment, the ignition point or flame front is maintainedwithin 2 cm. of the outlet port.

The present invention also provides a method where, during the exposingstep, cooling the substrate using a substrate cooling means. In apreferred embodiment, the substrate cooling means comprises a means fordirecting water onto the substrate. However, one of ordinary skill inthe art would recognize that many other suitable cooling means could beused.

In a further embodiment, the material that coats the substrate has athickness of less than 100 nm. In yet another embodiment, the materialthat coats the substrate comprises a graded composition. In anotherembodiment, the material that coats the substrate comprises an amorphousmaterial. In a further embodiment, the material that coats the substratecomprises a nitride, carbide, boride, metal or other non-oxygencontaining material.

The present invention also provides a method further comprising flowinga selected sheath gas around the reactable spray thereby decreasingentrained impurities and maintaining a favorable deposition environment.

In a preferred embodiment, the second selected pressure is above 20torr.

Referring now to FIG. 1, the preferred apparatus 100 comprises apressure regulating means 110, such as a pump, for pressurizing to afirst selected pressure a transport solution T (also called “precursorsolution”) in a transport solution reservoir 112, wherein the transportsolution T comprises a suitable carrier having dissolved therein one ormore reagents capable of reacting to form the selected material andwherein the means for pressurizing 110 is capable of maintaining thefirst selected pressure above the corresponding liquidus (if thetemperature is below T_(c)) or critical pressure, P_(c,), of thetransport solution T at the temperature of the transport solution T, afluid conduit 120 having an input end 122 in fluid connection with thetransport solution reservoir 112 and an opposed output end 124 having anoutlet port 126 oriented to direct the fluid in the conduit 120 into aregion 130 of a second selected pressure below the first selectedpressure and in the direction of the substrate 140, wherein the outletport 126 further comprises means 128 (see FIGS. 2 and 3, atomizer 4) fornebulizing a solution to form a nebulized solution spray N, atemperature regulating means 150 positioned in thermal connection withthe output end 124 of the fluid conduit 120 for regulating thetemperature of the solution at the output end 124 within 50° C. above orbelow the supercritical temperature, T_(c), of the solution, a gassupply means 160 for admixing one or more gases (e.g., oxygen) (notshown) into the nebulized solution spray N to form a reactable spray, anenergy source 170 at a selected energization point 172 for reacting thereactable spray whereby the energy source 170 provides sufficient energyto react the reactable spray in the region 130 of the second selectedpressure thereby coating the substrate 140.

In a further embodiment of the apparatus, the energy source 170comprises a flame source and the selected energization point 172comprises an ignition point. In an alternate embodiment, the energysource 170 comprises a plasma torch. In yet another embodiment, theoutlet port 126 further comprises a pressure restriction (see FIG. 3,restrictor 7).

In a further embodiment of the apparatus, the second selected pressureof the region is ambient pressure.

In yet another embodiment, the nebulized solution spray N is a vapor oran aerosol having a maximum droplet size of less than 2 μm.

In a further embodiment, the second selected pressure of the region isreduced to produce a combustion flame having a temperature of less than1000° C.

In yet another embodiment, the carrier is propane and the transportsolution comprises at least 50% by volume propane. In a furtherembodiment, the transport solution further includes butanol, methanol,isopropanol, toluene, or a combination thereof. In yet anotherembodiment, the carrier is selected such that the transport solution issubstantially precipitate free at standard temperature and pressure fora period of time sufficient to carry out the method.

In an alternate embodiment of the apparatus, a pressurized container(not shown) is provided and a standard temperature and pressure gas isalso contacted with the transport solution at a selected pressuresufficient to form a liquid or supercritical fluid. In a preferredembodiment, the transport solution containing the standard temperatureand pressure gas is substantially precipitate free at the selectedpressure for a period of time sufficient to carry out the method. In yetanother embodiment, the reagent concentration of the transport solutionis between 0.0005 M and 0.05 M.

In a further embodiment, the outlet end 124 of the conduit 120 furthercomprises a fluid introduction port (see FIG. 2, feed lines 17 or 19)and, prior to directing the pressurized, heated solution through theoutlet port 126 of the conduit 120, fluid is added to the pressurized,heated solution through the fluid introduction port. Such introductionforms a combined solution having a reduced supercritical temperature.

In yet another embodiment, each of the one or more reagents has a vaporpressure of no less than about 25% of the vapor pressure of the carrier.

In a further embodiment, the outlet end of the conduit comprises tubinghaving an internal diameter of 2 to 1000 μm, more preferably 10 to 250μm. In a more preferable embodiment, the outlet end of the conduitcomprises tubing having an internal diameter of 25 to 125 μm. In yet afurther preferable embodiment, the outlet end of the conduit comprisestubing having an internal diameter of 50 to 100 μm.

In another embodiment, the temperature regulating means 150 comprisesmeans for resistively heating the conduit by applying thereto anelectric current of a selected voltage from an electric current source.In a preferred embodiment, the voltage is less than 115 Volts. In yetanother preferred embodiment, the means for resistively heating theconduit comprises a contact 152 positioned within 4 mm of the outletport 126.

Moreover, it is provided that the above apparatus is utilized whereinthe carrier and one or more reagents are selected such that the secondselected temperature is ambient temperature.

The above apparatus may be used wherein the material that coats thesubstrate 140 comprises a metal. Alternatively, the material that coatsthe substrate 140 comprises one or more metal oxides. In yet a furtherembodiment, the material that coats the substrate 140 comprises at least90% silica.

In a further embodiment, the reactable spray comprises a combustiblespray having a combustible spray velocity and wherein the combustiblespray velocity is greater than the flame speed of the flame source atthe ignition point 172 and further comprising one or more ignitionassistance means 180 for igniting the combustible spray. In a preferredembodiment, each of the one or more ignition assistance means 180comprises a pilot light. In yet another embodiment, the combustiblespray velocity is greater than mach one.

In a further embodiment, the ignition point 172 or flame front ismaintained within 2 cm. of the outlet port.

The present invention also provides a substrate cooling means 190 forcooling the substrate 140. In a preferred embodiment, the substratecooling means 190 comprises a means for directing water onto thesubstrate 140. However, one of ordinary skill in the art would recognizethat many other suitable cooling means could be used.

In a further embodiment, the material that coats the substrate 140 has athickness of less than 100 nm. In yet another embodiment, the materialthat coats the substrate 140 comprises a graded composition.

There is further an apparatus provided comprising a means (see FIGS. 2and 3, feed line 17 or 19) for flowing a selected sheath gas around thereactable spray thereby decreasing entrained impurities and maintaininga favorable deposition environment.

In a preferred embodiment, the second selected pressure is above 20torr.

In a further embodiment of the method, the energy source comprises aflame source and the selected energization point comprises an ignitionpoint. In an alternate embodiment, the energy source comprises a plasmatorch, hot gasses, etc.

In a further preferred embodiment of the powder forming method, thetransport solution concentration is between 0.005 M and 5 M.

To simplify the operation, it is helpful to pump the precursor/solventsolution to the atomizing device at room temperature. Heating of thesolution should occur as a final step just prior to release of thesolution into the lower pressure region. Such late stage heatingminimizes reactions and immiscibilities which occur at highertemperatures. Keeping the solution below the supercritical temperatureuntil atomization maintains the dissolved amounts of precursor in theregion of normal solubility and reduces the potential of developingsignificant solvent-precursor concentration gradients in the solution.These solubility gradients are a result of the sensitivity of thesolution strength of a supercritical solvent with pressure. Smallpressure gradients (as they can develop along the precursor-solventsystem delivery) can lead to significant changes in solubility as hasbeen observed. For instance, the solubility of acridine in carbondioxide at 308° K. can be increased 1000 times by increasing thepressure from 75 atm to 85 atm. See V. Krukonis, “Supercritical FluidNucleation of Difficult to Comminute Solids”, Presented at AIChEMeeting, San Francisco, Nov. 25-30, 1984. Such solubility changes arepotentially detrimental because they may cause the precursor to bedriven out of solution and precipitate or react prematurely, clogginglines and filters.

The rapid drop in pressure and the high velocity at the nozzle cause thesolution to expand and atomize. For solute concentrations in the normalsolubility range, preferred for operation of the near supercriticalatomization system of the present invention, the precursors areeffectively still in solution after being injected into the low pressureregion. The term “effectively in solution” must be understood inconjunction with processes taking place when a solution with soluteconcentrations above the normal solvent strength is injected into thelow pressure region. In this case, the sudden pressure drop causes highsupersaturation ratios responsible for catastrophic solute nucleationconditions. If the catastrophic nucleation rapidly depletes the solventfrom all dissolved precursor, the proliferation of small precursorparticles is enhanced. See D. W. Matson, J. L. Fulton, R. C. Petersenand R. D. Smith, “Rapid Expansion of Supercritical Fluid Solutions:Solute Formation of Powders, Thin Films, and Fibers”, Ind. Eng. Chem.Res., 26, 2298 (1987); H. Anderson, T. T. Kodas and D. M. Smith, “VaporPhase Processing of Powders: Plasma Synthesis and AerosolDecomposition”, Am. Ceram. Soc. Bull., 68, 996 (1989); C. J Chang and A.D Randolph, “Precipitation of Microsize Organic Particles fromSupercritical Fluids”, AIChE Journal, 35, 1876 (1989); T. T. Kodas,“Generation of Complex Metal Oxides by aerosol Processes:Superconducting Ceramic Particles and Films”, Adv. Mater., 6, 180(1989); E. Matijevic, “Fine Particles: Science ad Technology”, MRSBulletin, 14, 18 (1989); E. Matijevic, “Fine Particles Part II:Formation Mechanisms and Applications”, MRS Bulletin, 15, 16 (1990); R.S. Mohamed, D. S. Haverson, P. G. Debenedetti and R. K. Prud'homme,“Solid Formation After Expansion of Supercritical Mixtures,” inSupercritical Fluid Science and Technology, edited by K. P. Johnston andJ. M. L. Penniger, p.355, American Chemical Society, Washington, D.C.(1989); R. S. Mohamed, P. G. Debeneletti and R. K. Prud'homme, “Effectsof Process Conditions on Crystals Obtained from Supercritical Mixtures”,AlChE J., 35, 325 (1989); J. W. Tom and P. G. Debenedetti, “Formation ofBioerodible Polymeric Microspheres and Microparticles by Rapid Expansionof Supercritical Solutions”, Biotechnol. Prog., 7, 403 (1991). Particlesare undesirable for the formation of thin coatings, but can bebeneficial during the formation of powders.

Thus the heated atomizer provides the further superior advantages,compared to an unheated device that operates on rapid expansion of asolvent at exclusively above the supercritical temperature, that (1) thetemperature allows for a well controlled degree of atomization of theprecursor-solvent mixture and (2) catastrophic nucleation of theprecursors can be omitted while still enjoying the benefits ofsupercritical atomization. Supersonic velocities can be created forminga mach disk which additionally benefits atomization. Addition of gassesto the released atomized materials aids in directing the flow and canensure a desired mixture for combustion.

By adjusting the heat input into the atomizing device, the liquidsolution can be vaporized to various degrees. With no heat input to theatomizing device, liquid solutions of higher supercritical temperatureliquids, that are liquids at STP, can exit in the form of a liquidstream which is clearly far from a supercritical condition. This resultsin a poorly formed flame and, possibly, undesirable liquid contact withthe substrate. Decreasing the temperature differential of the liquidsolution to its supercritical temperature at the nozzle causes theliquid solution to break up into droplets forming a mist which isreleased from the atomizing device. The droplets vaporize, and thusbecome invisible, after a short distance. As the supercriticaltemperature at the atomizing device is approached, the liquid solutiondroplets decrease in size, and the distance to solution vaporization isdecreased. Using this atomizer the vapor droplet size was determinedusing an aerosol vaporization tester and the obtained droplet size wasbelow the 1.8 μm detection limit of the instrument.

Further increasing the heat input results in a state of no mist at thetip, or complete vaporization. Without wishing to be bound by theory,this behavior of the solution can be attributed to the combinedsupercritical properties of the reagents and solvents. Solutions ofprecursors in lower supercritical temperature solvents, that are gassesat STP, behave similarly, but the emerging solution from the tip (alsoreferred to as the “nozzle” or “restrictor”) does not form a liquidstream, even without heat input. The amount of heat needed to obtainoptimal vaporization of the solution depends mostly on the heat capacityof the solution and the differential between the supercriticaltemperature of the solvent and the ambient temperature around thenozzle.

It is desirable to maintain the pressure and temperature of the system(before vaporization) above the boiling and the supercritical point ofthe solution. If the pressure falls below the liquidus or criticalpressure, coincident with the temperature above the boiling point,vaporization of the solvents will occur in the tube prior to the tip.This leaves the solutes which can build up and clog the atomizingdevice. Similarly the pressure is preferably sufficiently high in thesupercritical region so that the fluid is more liquid-like. Liquid-likesupercritical fluids are better solvents than more gas-likesupercritical fluids, further reducing the probability of solutesclogging the atomizing device. If the precursor-to-precursor interactionis higher than the strength between solvent and precursor, thesolvent-precursor bonds can be broken and effectively drive theprecursor out of solution. Precursor molecules then form clusters thatadhere to the atomizing device and clog the restrictor. The problem canbe solved, in most cases, by shifting the vaporization point from theinside of the tip to the end of the tip, which is accomplished byreducing the heat input into the atomizing device. Another solution isto use a solvent which forms stronger bonds with the precursor so a morestable solution is formed. A small amount of mist at the tip usuallyresults in the best quality thin films. Nano- or micro-spheres of thematerial will form if the temperature of the solution it too high or toolow. These spheres are detrimental if dense coatings are desired.

If the no-mist condition is reached, the deposition is being performedabove the critical temperature. The heat of the flame and mixing withexternal gasses keeps STP liquid solvents from condensing and formingdroplets. In the no-mist instance, atomization and intermixing is verygood but flow stability is reduced, resulting in a flame that can jumpfrom side to side with respect to the direction of the tip. With such aflame behavior, depositions remain possible, but it can be difficult todeposit films requiring stringent thickness uniformity. Additionally, itis necessary to maintain the temperature of the solution, prior torelease, below the temperature where either the solute precipitates orreacts and precipitates. When using a solvent mixture it may be possibleduring heating to cross the line for spinoidal immiscibility. Thiscauses the formation of two separate phases, with the possibility ofconcentration differences in the two phases due to differentsolubilities of the solutes. This may influence the formation ofprecursor and product spheres at high atomization temperatures. All ofthese factors demonstrate the preferability of minimizing the solution'sexposure to heating, if necessary, until the tip so that possibleunwanted equilibrium condition states of matter do not have sufficienttime to transpire. The structure of the films deposited can thus beprecisely controlled.

Due to this control, a number of film microstructures are possible. Byincreasing solution concentration it is possible to increase thedeposition rate and the following microstructural changes result withincreasing solution concentration; dense to porous, specular to dull,smooth to rough, columnar to hillocks, and thin to thick. Graded andmultilayered coatings can also be produced. Multilayers can be formed bysupplying different precursor containing solutions to an individualflame. Sequential multiple deposition flames may be used to increasethroughput for production applications. Some additional factorscontrolling deposition parameters include; substrate surface temperaturewhich controls surface diffusion and nucleation; pressure which controlsboundary layer thickness and thus deposition rate, solution compositionand mix gasses varies the material being deposited and thus the coatingsgrowth habit, flame and plasma energy level effects where the reactionoccurs and vapor stability, and the distance to the substrate effectsthe time from nebulization to reaction to deposition which can lead toparticle formation or increased diffusion time for larger clusters.Additionally, electric and magnetic fields affect the growth habits ofsome materials, or increase deposition efficiency. One of ordinary skillin the art would recognize that such electric and magnetic fields willaffect the growth habits of some vapor deposited materials, as well asvary the particular deposition rate and efficiency.

Because the required energy input into the solution heating atomizervaries for different precursor/primary-solvent/secondary-solventsolutions, it is preferred to deposit multilayer thin films fromsolutions with constant primary to secondary solvent ratios. In sodoing, it is not necessary to change the energy input to the atomizerwhen switching from one solution to another solution. The resultingsimplification of the setup produces increased performance andreliability while reducing costs. Alternatively, the substrate can bepassed by flames containing different reagents to build the desiredmultilayer.

When the solution provides the fuel for combustion, concentrations up to0.1 molar result in dense coatings depending on the material. Mostmaterials have preferred concentrations of up to 0.01 molar. Materialswith lower diffusion and mobility need solution concentrations of lessthan 0.002. Solution concentrations of less than 0.0001 molar result invery slow deposition rates for most materials. Flame depositions withadded combustible materials can have higher concentrations, evenexceeding 1 M, but for the preferable vapor formation of the precursors,high concentrations are less desirable unless the precursor(s) have highvapor pressures. Low vapor pressure precursor solution concentrationsare preferably less than 0.002 molar.

Without wishing to be bound by theory, it is helpful to understand thatthe principle of the deposition technique of the present inventioninvolves the finding that CVD its not limited to reactions at thesurface. See Hunt, A. T., “Combustion Chemical Vapor Deposition, a NovelThin Film Deposition Technique”, Ph.D. Thesis Georgia Inst. of Tech,Atlanta, Ga., (1993); Hunt, A. T., “Presubstrate Reaction CVD, and aDefinition for Vapor”, presented at the 13th Int. Conf. on CVD, LosAngles, Calif. (1996), the contents of which are hereby incorporated bythis reference. Reactions can occur predominately in the gas stream, butthe resulting material which is deposited must be subcritical in size toyield a coating with vapor deposited microstructures. These observationsdemonstrate that a vapor is composed of individual atoms, molecules ornanoclusters which can be absorbed onto a substrate and readily diffusedinto lower energy sites or configurations. Thus the maximum cluster sizemust decrease with lower substrate temperatures as does the criticalnucleus size. It is known by one of ordinary skill in the art thatreagent clusters are left after vaporization of the solvents, and thecluster size is related to the reagent vapor pressure, initial dropletsize and the solution concentration. Therefore, atomization of low vaporpressure reagents, which therefore do not gasify in the flame, must bevery fine to form vapor.

Preferred liquid solvents are low cost solvents include, but are notlimited to, ethanol, methanol, water, isopropanol and toluene. Watersolutions must be fed into a preexisting flame, while the combustiblesolvents can themselves be used to form the flame. It is preferable, butnot required, to form the bulk of the flame using the solution ratherthan feeding the solution into a flame. Lower reagent concentrationresults this way, which eases the formation of subcritical nucleus sizedmaterials.

One preferred solvent and secondary solution fluid which is propane,which is a gas at STP. However, it must be noted that many other solventsystems are operable. See, e.g., CRC Handbook of Chemistry and Physics,CRC Press, Boca Raton, Fla. Propane is preferred because of its lowcost, its commercial availability, and its safety. Many low costorganometalics can be used in a predominately propane solution. To easehandling, the initial precursors can be dissolved in methanol,isopropanol, toluene or other solvents compatible with propane. Thisinitial solution is then placed into a container into which liquidpropane is added. Propane is a liquid at above only about 100 psi atroom temperatures. The resulting solution has a much lower supercriticalpoint than the initial solution which eases atomization by lowering therequired energy input into the atomizer. Additionally, the primarysolvent acts to increase the polar solubility of the propane, thusallowing higher solution concentrations for many reagents than wouldotherwise be achieved by propane alone. As a general rule, the polarityof the primary solvent should increase with increasing polarity of thesolute (precursor). Isopropanol can thus aid in the solubility of apolar solute better than toluene. In some cases the primary solvent actsas a shield between the secondary solvent and a ligand on the solute.One example is the dissolution of platinum-acetylacetonate[Pt(CH₃COCHCOCH₃)₂] in propane, where the weight ratios betweenprecursor/primary solvent and primary solvent/secondary solvent can behigher than those required in other systems.

Ammonia has been considered and tested as a secondary solvent for thedeposition of coatings and powders. While ammonia is an inexpensivesolvent that is compatible with some nitrate based precursors, it is noteasily usable with other secondary solvents and problems stem from thegeneral aggressiveness of pure ammonia. The atomization properties ofammonia were tested without the addition of a precursor and the usedpressure vessel was significantly attacked after the experiment evenwhen an inert Type-316 stainless steel vessel was used. In contrast tohydrocarbon based solvents, ammonia also renders Buna-N and Vitongaskets useless after only a few minutes. Even with a suitable gasketmaterial this is a problem since the desired coatings or powders usuallymust not contain traces of iron or other elements leached from thepressure vessel wall. However, there are materials, such as EPDMelastomer which may be used.

Other gas-like secondary solvents that were tested and can be usedinclude ethane, ethylene, ethane/ethylene mixture, propane/ethylenemixture, and propane/ethane mixture. Platinum thin films were depositedfrom a supercritical mixture of ethane and a platinum metalorganic.

Other tested solvents and solvent mixtures resulted in similar quality,but were more complex to work with since their boiling points aresignificantly lower, which required cooling of the solution or very highpressures. The ease of handling makes propane the preferred solvent butthe other supercritical solvents are considered alternatives to propanein cases where propane cannot be used, such as when a precursor that issoluble in propane cannot be found. Other fluids can be used to furtherreduce the supercritical temperature if desired.

One heating method is the application of an electric current between thenozzle end, where the precursor solution is injected into the lowpressure region, and the back of the restriction tube. This directlyheated restrictive tube method allows for fast changes in atomizationdue to a short response time. The location of most intense heating canbe shifted toward the tip by increasing the connection resistancebetween the tip and the electrical lead connected to the tip. Thinwalled restriction tubes possess a larger resistance than thick walledtubes and decrease the response time. Other heating methods can beapplied and several have been investigated, including but not limitedto, remote resistive heating, pilot flame heating, inductive heating andlaser heating. One of ordinary skill in the art could readily determineother suitable heating means for regulating the temperature at theoutlet port of the atomizer.

Remote resistive heating uses a non-conducting restriction tube that islocated inside an electrically heated tube. The non-conducting tube willfit tightly into the conductive tube. Application of an electric currentto the conductive type heats that tube and energy is transferred intothe inner, non-conductive restriction tube. This method requires largerheating currents compared to the directly-heated restrictive tube methodand shows longer response times, which can be advantageous under certainconditions since the increased response time results in a high degree ofthermal stability. On the other hand, pilot flame and laser heating usethe energy of the pilot flame or laser light, respectively, to heat therestriction tube. This can be done in a directly heated setup where thetip of the restriction tube is subjected to the pilot flame or laserlight or in an indirectly heated configuration where the larger outertube is heated. Because the amount of energy that needs to betransferred into the solution is quite large, the heated tube will,preferably, have a thicker wall than in the case of direct electricalheating or remote electrical heating. Subjecting an outer tube to thepilot flame or laser light allows the use of a thin walled restrictiontube.

Referring now to FIGS. 2 and 3, an apparatus 200 for the deposition offilms and powders using supercritical atomization is shown. Theapparatus 200 consists of a fixed or variable speed pump 1 that pumpsthe reagent transport solution 2 (also called “precursor solution”) fromthe solution container 3 into the atomizer (also referred to as the“nebulizer” or “vaporizer”) 4. FIG. 3 is an inset view showing a moredetailed schematic view of the atomizer 4. The precursor solution 2 ispumped from the precursor solution container 3 through lines 5 andfilters 6 and into the atomizer 4. The precursor solution 2 is thenpumped into a constant or variable temperature controlled restrictor 7.Heating can be accomplished in many ways including, but not limited to,resistive electrical heating, laser heating, inductive heating, or flameheating. For resistive electrical heating, either AC or DC current canbe used. One of the electrical connections 8 to the restrictor 7 ispreferably placed very close to the tip of the restrictor 7. In the caseof heating by a DC source, this connection 8 or pole can be eitherpositive or negative. The other pole 9 can be connected at any otherpoint along the restrictor 7, inside or outside the housing 10. Forspecial applications such as coating the inside of tubes, where a smalltotal atomizer size is advantageous, it is preferable to either connectto the restrictor 7 at the back of the housing 10 or to connect insidethe housing 10. Gas connections at the back of the housing 10 are shownin an on-line arrangement but can be placed in any other arrangementthat does not interfere with the function of the apparatus 200.

The thin gas A supply line 11, {fraction (1/16)}″ ID in most cases,carries a combustible gas mix to a small outlet 12 where it can serve asa stable pilot flame, preferably within 2.5 cm of the restrictor 7, forthe combustion of the precursor solutions supplied via the restrictor 7.Gas A supply is monitored by a flow controller 13, controlling the flowof the individual gas A mix components, 14 and 15. The gas A fuelcomponent 14 is mixed with the oxidizing component 15 in a mixing “T” 16close to or inside the atomizer 4. This late mixing is preferably forsafety reasons because it reduces potential flash-back. Distributionchannels inside the housing 10 connect the gas supply lines 11 to thegas A feed 17. Gas B supply lines 18 are used to deliver gas B from thesupply 19 such that good mixing with the nebulized solutions spray canbe accomplished. In most cases a high velocity gas stream is utilized. Anumber of gas B supply holes 20 (six for most cases, more or less holescan be used depending on the particular application) is placed aroundthe restrictor 7 supplying gas B such that the desired flow pattern isobtained. The flow properties of the gas B stream are influenced by suchfactors as gas B pressure in the gas B storage container 21, flow rateas determined by the flow controller 13, line diameters 5, and number ofsupply holes 20. Alternatively, gas B can be fed through a larger tubecoaxial to and surrounding the restrictor 7. Once the precursor solution2 has been pumped into the precursor supply 22 its temperature iscontrolled by the current flow (in the case of electrical heating)through the restrictor 7 as determined by the power supply 23. Thisheating current can then be adjusted such that the proper amount ofatomization (nebulization, vaporization) can occur. The stable pilotflame is then capable of igniting the nebulized reactive spray anddepositing a powder or film on a substrate 24.

Many different coatings have been deposited using the methods andapparatuses described herein. While propane was used in most cases asthe super critical secondary solvent (i.e. a small amount of highprecursor concentration primary solvent was mixed with a large amount ofsecondary solvent), others solvents have been used. Other possiblesecondary solvents include, but are not limited to N₂O, ethylene,ethane, and ammonia.

One of ordinary skill in the art would recognize that almost anysubstrate can be coated by the method and apparatus of the presentinvention. A substrate can be coated if it can withstand the temperatureand conditions of the resulting hot gases produced during the process.Substrates can be cooled using a means for cooling (described elsewhereherein), such as a water jet, but at low substrate surface temperatures,dense or crystalline coatings of many materials are not possible becauseof the associated low diffusion rates. In addition, substrate stabilityin the hot gases can be further accounted for by using a lowtemperature, low pressure flame, either with or without additionalsubstrate cooling.

A variety of chemical precursors have been suggested for CCVD depositionof films and powders, and additional chemical precursors are suggestedherein. In addition to providing the metal or metalloid element, it isrequired of any chemical precursor for CCVD that it be soluble in asuitable carrier solvent, most desirably soluble in propane.Furthermore, if the precursor solution is to contain precursors of morethan one metal and/or metalloid, the chemical precursors must bemutually soluble in a suitable carrier solvent and chemically compatiblewith each other. If a precursor is not highly soluble in a primarysolvent, such as propane, it may be initially dissolved in a secondarysolvent, such as toluene, and subsequently introduced into the primarysolvent as a solution in the secondary solvent, providing that thechemical precursor does not precipitate when such a solution isintroduced into the primary solvent. Furthermore, cost considerationsenter into the choice of chemical precursor.

If a mixture of chemical precursors are to be provided for depositing alayer or powder of a particular composition, it is desirable that suchprecursors be combinable as a homogeneous “pre-solution” without theaddition of any additional solvent. If not, it is desirable that allchemical precursors be mutually soluble in a common solvent, the lesssolvent the better, as a “pre-solution”. These desired properties, ofcourse, facilitate shipping and handling, particularly when the intendedprimary solvent is propane or another material which is gaseous at roomtemperature. Though desirable to be able to provide a “pre-solution”, itis considered acceptable that the chemical precursors be mutuallysoluble in a deposition solution of one or more solvents and either beprepared and sold as such a solution or prepared on-site as a depositionsolution.

For deposition, the total concentration of the precursor compounds inthe carrier solvent is generally between about 0.001 and about 2.5 wt %,preferably between about 0.05 and about 1.0 wt %.

For most CCVD depositions, it is preferred that the precursors bedissolved in an organic solvent. However, for the electrically resistivematerials to which the present invention is directed, it is undesirablethat carbon co-deposits with the resistive material. Some materials,nickel, for example, have a high affinity for carbon. Accordingly,precursors for such materials may be preferably dissolved in an aqueousand/or ammonia solution, in which case, the aqueous and/or ammoniaand/or N₂O solution would be aspirated into a hydrogen/oxygen flame forCCVD.

One of the advantages of CCVD, as performed with preferred atomizingapparatus, relative to other deposition methods, is that the a precursorsolution containing one or more dissolved chemical precursors isatomized as a near-super critical liquid or, in some cases, as a supercritical fluid. Accordingly, the amount of precursor or precursors beingburned and deposited on a substrate or deposited in powder form isindependent of the relative vapor pressures of the individual chemicalprecursors and the carrier solvent or solvents. This is in contrast toconventional CVD processes where individual supply lines must beprovided for each chemical precursor that is to be vaporized, generallywithin a carrier gas, for supply to a CVD furnace. Also, someconventional CVD precursors disproportionate, making it difficult tosupply such a chemical precursor uniformly—another problem readilyaddressed by CCVD technology.

A Controlled Atmosphere Combustion Chemical Vapor Deposition (CACCVD)apparatus is illustrated in FIGS. 7 and 8. A coating precursor 710 ismixed with a liquid media 712 in a forming zone 714, comprising a mixingor holding tank 716. The precursor 710 and liquid media 712 are formedinto a flowing stream which is pressurized by pump 718, filtered byfilter 720 and fed through conduit 722 to an atomization zone 724, fromwhich it flows successively through reaction zone 726, deposition zone728 and barrier zone 730. It is not required that a true solution beformed from mixing the coating precursor 710 with the liquid media 712,provided the coating precursor is sufficiently finely divided in theliquid media. However, the formation of a solution is preferred, since,generally, such produces a more homogeneous coating.

The flowing stream is atomized as it passes into the atomization zone724. Atomization can be accomplished by recognized techniques foratomizing a flowing liquid stream. In the illustrated apparatus,atomization is effected by discharging a high velocity atomizing gasstream surrounding and directly adjacent the flowing stream as itdischarges from conduit 722. The atomizing gas stream is provided from agas cylinder or other source of high pressure gas. In the illustratedembodiment, high pressure hydrogen (H₂) is used both as an atomizing gasand as a fuel. The atomizing gas is fed from hydrogen gas cylinder 732,through regulating valve 734, flowmeter 736 and into conduit 738.Conduit 738 extends concentrically with conduit 722 to the atomizationzone where both conduits end allowing the high-velocity hydrogenatomizing gas to contact the flowing liquid stream thereby causing it toatomize into a stream of fine particles suspended in the surroundinggas/vapors. This stream flows into the reaction zone 726 wherein theliquid media vaporizes and the coating precursor reacts to form areacted coating precursor, which often involves dissociation of thecoating precursor into ions of its components and results in a flowingstream of ionic particles, or plasma. The flowing stream/plasma, passesto the deposition zone 728 wherein the reacted coating precursorcontacts the substrate 740 depositing the coating thereon.

The flowing stream may be atomized by injecting the atomizing gas streamdirectly at the stream of liquid media/coating precursor as it exitsconduit 722. Alternatively, atomization can be accomplished by directingultrasonic or similar energy at the liquid stream as it exits conduit722.

The vaporization of the liquid media and reaction of the coatingprecursor require substantial energy input to the flowing stream beforeit leaves the reaction zone. This energy input can occur as it passesthrough the conduit 722, or in the atomization and/or reaction zones.The energy input can be accomplished by a variety of known heatingtechniques, such as electrical resistance heating, microwave or RFheating, electrical induction heating, radiant heating, mixing theflowing stream with a remotely heated liquid or gas, photonic heatingsuch as with a laser, etc. In the illustrated preferred embodiment, theenergy input is accomplished by the combustion of a fuel and an oxidizerin direct contact with the flowing stream as it passes through thereaction zone. This relatively new technique, referred to as CombustionChemical Vapor Deposition (CCVD), is more fully described in theincorporated U.S. Pat. No. 5,652,021. In the illustrated embodiment, thefuel, hydrogen, is fed from hydrogen gas cylinder 732, through aregulating valve, flowmeter 742 and into conduit 744. The oxidizer,oxygen, is fed from oxygen gas cylinder 746, through regulating valve748 and flowmeter 750 to conduit 752. Conduit 752 extends about andconcentric with conduit 744, which extends with and concentrically aboutconduits 722 and 738. Upon exiting their respective conduits, thehydrogen and oxygen combust creating combustion products which mix withthe atomized liquid media and coating precursor in the reaction zone726, thereby heating and causing vaporization of the liquid media andreaction of the coating precursor.

A curtain of a flowing inert gas provided around at least the initialportion of the reaction zone isolates the reactive gases from thematerials present in the apparatus located in proximity to the reactionzone. An inert gas, such as argon, is fed from inert gas cylinder 754,through regulating valve 756 and flowmeter 758 to conduit 760. Conduit760 extends about and concentric with conduit 752. Conduit 760 extendsbeyond the end of the other conduits 722, 738, 744 and 752, extendingclose to the substrate whereby it functions with the substrate 740 todefine a deposition zone 728 where coating 762 is deposited on thesubstrate generally in the shape of the cross-section of conduit 760. Asthe inert gas flows past the end of oxygen conduit 752, it initiallyforms a flowing curtain which extends about the reaction zone, shieldingthe reactive components therein from conduit 760. As it progresses downthe conduit 760, the inert gas mixes with the gases/plasma from thereaction zone and becomes part of the flowing stream directed to thedeposition zone 728.

An ignition source is needed to initially ignite the hydrogen andoxygen. A separate manually manipulated lighting or ignition device issufficient for many applications, however the use of such may require atemporary reduction in the flow of inert gas until a stable flame frontis established. In some applications, the total flow of gas may be toogreat to establish an unassisted stable flame front. In such case, it isnecessary to provide an ignition device capable of continuously orsemi-continuously igniting the combustible gases as they enter thereaction zone. A pilot flame or a spark producing device are exemplaryignition sources which may be employed.

In the deposition zone 728, the reacted coating precursor depositscoating 762 on the substrate 740. The remainder of the flowing streamflows from the deposition zone through a barrier zone 730 to dischargeinto the surrounding, or ambient, atmosphere. The barrier zone 730functions to prevent contamination of the deposition zone by componentsof the ambient atmosphere. The high velocity of the flowing stream as itpasses through the barrier zone 730 is a characteristic feature of thiszone. By requiring that the flowing stream achieve a velocity of atleast fifty feet per minute as it passes through the barrier zone, thepossibility of contamination of the deposition zone by components of theambient atmosphere is substantially eliminated in most coatingapplications. By requiring that the flowing stream achieve a velocity ofat least one hundred feet per minute the possibility of ambientatmosphere contamination of the deposition zone is essentiallyeliminated in those coating operations which are more highlycontamination sensitive, such as the production of TiN or WC.

In the embodiment of FIG. 7, a collar 764 is attached to and extendsperpendicularly outward from the end of conduit 760 adjacent depositionzone 728. The barrier zone 730 is defined between the collar 764 and thesubstrate 740. The collar is shaped to provide a conforming surface 766deployed close to the surface of the substrate whereby a relativelysmall clearance is provided for the exhaust of gases passing from thedeposition zone to the ambient atmosphere. The clearance establishedbetween the conforming surface 764 of the collar and the substrate issufficiently small that the exhaust gases are required to achieve thevelocity required in the barrier zone for at least a portion of theirpassage between the collar and the substrate. To this end, theconforming surface 764 of the collar 762 is shaped to lie essentiallyparallel to the surface of the substrate 740. When the surface of thesubstrate 740 is essentially planar, as it is in the illustratedembodiment, the conforming surface of the substrate is alsosubstantially planar.

Edge effects, such as elevated temperatures and residual reactivecomponents, which occur adjacent the end of the conduit 760 can extendthe deposition zone beyond the area of the substrate directly in frontof the end of conduit 760. The collar 764 should extend outward from itsjoinder to the conduit 760 a sufficient distance to preclude theback-mixing of ambient gases into the deposition zone due to a possibleVenturi effect, and to assure that the entire area of the depositionzone, as it is extended by the previously noted edge effects, isprotected from the backflow of ambient gases by the high velocityexhaust gases sweeping through the area between the collar and thesubstrate. The extended collar assures that contamination is preventedthroughout the extended deposition zone. The diameter of the collarshould be at least twice the internal diameter of conduit 760, andpreferably, should be at least five times the internal diameter ofconduit 760. The internal diameter of conduit 760 typically is in therange of 10 to 30 millimeters, and preferably is between 12 and 20millimeters.

In operation, the collar 764 is located substantially parallel to thesurface of the substrate 740 being coated and at a distance therefrom of1 centimeter or less. Preferably, the facing surfaces of the collar andthe substrate are between 2 and 5 millimeters apart. Spacing devices,such as three fixed or adjustable pins (not shown), may be provided onthe collar to assist in maintaining the proper distance between thecollar and the substrate.

The embodiment illustrated in FIG. 7 is particularly advantageous forapplying coatings to substrates which are too large, or for which it isnot convenient, to be treated in a specially controlled environment suchas a vacuum chamber or a clean room. The illustrated coating techniqueis advantageous because it can be accomplished under atmosphericpressure conditions and at more convenient “in the field” locations. Theseries of concentric conduits 722, 738, 744, 752 and 760 form a coatinghead 768 which can be supplied by relatively small flexible tubes andcan be sufficiently small to be portable. Large substrates can be coatedeither by having the coating head traverse the substrate repeatedly in araster or similar pattern, or by traversing the substrate with an arrayof coating heads arranged to cumulatively provide a uniform coating, orby rastering an array of coating heads. In addition to permitting thethin film coating of articles which previously were too large to becoated, this technique permits the coating of larger units of thosesubstrates which previously were coated under vacuum conditions.Manufacturing economies can be achieved by coating larger units of thesesubstrates, especially when mass production of the substrates isinvolved.

The embodiment illustrated in FIGS. 7 and 8 is also particularlysuitable for the production of coatings which are oxidation sensitive,such as most metal coatings. To provide such coatings the fuel is fedthrough conduit 744 in proximity to the atomized liquid media andcoating precursor, while the oxidizer is fed through conduit 752. Theatomizing gas fed through conduit 738 and/or the liquid media fedthrough conduit 722 can be materials having fuel value, they can bematerials which react with the coating precursor or they can be inertmaterials. When the produced coatings or coating precursor materials areoxygen sensitive, a reducing atmosphere is maintained in the reactionand deposition zones by assuring that the total amount of oxidizer fedis restricted to an amount less than that required to fully combust thefuel provided to the reaction zone, i.e. the oxidizer is provided inless than stoichiometric amount. Generally, the fuel excess is limitedso as to limit any flame zone which develops when the residual hot gasesmix with atmospheric oxygen. When the produced coatings and theprecursor materials are oxygen-tolerant or enhanced by the presence ofoxygen, such as in the production of most of the oxide coatings, anoxidizing or neutral atmosphere may be provided in the reaction anddeposition zones by feeding a stoichiometric or excess amount ofoxidizer. Further, with oxygen tolerant reagents and products, theoxidizer can be fed through the inner conduit 744 while fuel is fedthrough outer conduit 752.

The inert gas supplied through conduit 760 must be sufficient to shieldthe inside surface of the conduit from the reactive gases produced inthe reaction zone, and it must be sufficient, when added with the othergases from the reaction zone, to provide the gas velocity required inthe barrier zone.

The energy input can be accomplished by mechanisms other than thecombustion method illustrated in FIGS. 7 and 8. For instance, it couldbe accomplished by passing electrical current through conduit 722 tocreate electrical resistance heat in the conduit which then transfers tothe liquid medium and coating precursor as it passes through theconduit. It should be apparent that all of the conduits 722, 738, 744,752 and 760 are not required when the energy input is accomplished byother than the combustion method. Usually one or both of conduits 744and 752 are omitted when the energy input is provided by one of theelectrically derived energy input mechanisms.

The porosity or density of the deposited coating can be modified byvarying the distance between the flame zone and the deposition zone atthe substrate's surface. Generally, shortening of this distance providesan increased coating density, while increasing the distance provides amore porous coating.

In the illustrated CACCVD technique the reaction zone is generallycoextensive with the flame produced by the burning fuel. Of course, theflame zone and the substrate must be maintained sufficiently far apartthat the substrate is not damaged by the higher temperatures which wouldresult as the flame zone more closely approaches the substrate surface.While substrate temperature sensitivity varies from one substratematerial to the next, the temperature in the deposition zone at thesubstrate surface, typically, is at least 600° C. cooler than themaximum flame temperature.

When some of the alternate methods are used to supply the energy input,such as when the principal energy input is a preheated fluid which ismixed with the flowing stream in, or before it reaches, the reactionzone, the maximum temperatures produced in the reaction zone aresubstantially lower than those produced with a combustion energy input.In such cases the coating properties can be adjusted by varying thedistance between the reaction zone and at the substrate surface withless concern for overheating the substrate. Accordingly, the termsreaction zone and deposition zone are useful in defining functionalregions of the apparatus but are not intended to define mutuallyexclusive regions, i.e. in some applications reaction of the coatingprecursor may occur in the deposition zone at the substrate surface.

The lower maximum temperatures resulting when the principal energy inputis other than a combustion flame enables the use of temperaturesensitive coating materials, such as some organic materials. Inparticular, polymers may be deposited as protective coatings or asdielectric interlayer materials in capacitors, integrated circuits ormicroprocessors. For instance, a polyimide coating could be providedfrom its polyamic acid precursor. Similarly, polytetrafloroethylenecoatings could be provided from low molecular weight precursors.

The energy input to the flowing stream prior to its leaving the reactionzone generally negates the need to provide energy to the deposition zoneby heating the substrate, as is often required in other coatingtechniques. In the present deposition system, since the substrate actsas a heat sink to cool the gases present in the deposition zone, ratherthan heating them, the temperatures to which the substrates aresubjected are substantially less than are encountered in systems whichrequire that energy be transmitted to the deposition zone through thesubstrate. Accordingly, the CACCVD coating process can be applied tomany temperature sensitive substrate materials which can not be coatedby techniques which involve heating through the substrate.

A wide range of precursors can be used as gas, vapor or solutions. It ispreferred to use the lowest cost precursor which yields the desiredmorphology. Suitable chemical precursors, not meant to be limiting, fordepositing various metals or metalloids are as follows:

Pt platinum-acetylacetonate [Pt(CH₃COCHCOCH₃)₂] (in toluene/methanol),platinum-(HFAC₂), diphenyl-(1,5-cyclooctadiene) Platinum (II) [Pt(COD)in toluene-propane]platinum nitrate (in aqueous ammonium hydroxidesolution)

Mg Magnesium naphthenate, magnesium 2-ethylhexanoate[Ma(OOCCH(C₂H₅)C₄H₉)₂], magnesium naphthenate, Mg-TMHD, Mg-acac,Mg-nitrate, Mg-2,4-pentadionate

Si tetraethoxysilane [Si(OC₂H₅)₄], tetramethylsilane, disilicic acid,metasilicic acid

P triethyl phosphate [C₂H₅O)₃PO₄], triethyllphosphite, triphenylphosphite

La lanthanum 2-ethylhexanoate [La(OOCCH(C₂H₅)C₄H₉)₃] lanthanum nitrate[La(NO₃)₃], La-acac, La-isopropoxide, tris(2,2,6,6-tetramethyl-3,5-heptanedionato), lanthanum [La(C₁₁CH₁₉O₂)₃]

Cr chromium nitrate [Cr(NO₃)₃], chromium 2-ethylhexanoate[Cr(OOCCH(C₂H₅)C₄H₉)₃], Cr-sulfate, chromium carbonyl, chromium(III)acetylacetonate

Ni nickel nitrate [Ni(NO₃)₂] (in aqueous ammonium hydroxide),Ni-acetylacetonate, Ni-2-ethylhexanoate, Ni-napthenol, Ni-dicarbonyl

Al aluminum nitrate [Al(NO₃)₃], aluminum acetylacetonate[Al(CH₃COCHCOCH₃)₃], triethyl aluminum, Al-s-butoxide, Al-i-propoxide,Al-2-ethylhexanoate

Pb Lead 2-ethylhexanoate [Pb(OOCCH(C₂H₅)C₄H₉)₂], lead naphthenate,Pb-TMHD, Pb-nitrate

Zr zirconium 2-ethylhexanoate [Zr(OOCCH(C₂H₅)C₄H₉)₄], zirconiumn-butoxide, zirconium (HFAC₂), Zr-acetylacetonate, Zr-n-propanol,Zr-nitrate

Ba barium 2-ethylhexanoate [Ba(OOCCH(C₂H₅)C₄H₉)₂], Ba-nitrate,Ba-acetylacetonate, Ba-TMHD

Nb niobium ethoxide, tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionato)niobium

Ti titanium (IV) i-propoxide [Ti(OCH(CH₃)₂)₄], titanium (IV)acetylacetonate, titanium-di-i-propoxide-bis-acetylacetonate,Ti-n-butoxide, Ti-2-ethylhexanoate, Ti-oxide bis(acetylacetonate)

Y yttrium 2-ethylhexanoate [Y(OOCCH(C₂H₅)C₄H₉)₃], Y-nitrate,Y-i-propoxide, Y-napthenoate

Sr strontium nitrate [Sr(NO₃)₂], strontium 2-ethylhexanoate, Sr(TMHD)

Co cobalt naphthenate, Co-carbonyl, Co-nitrate,

Au chlorotriethylphosphine gold (I), chlorotriphenylphosphine gold(I)

B trimethylborate, B-trimethoxyboroxine

K potassium ethoxide, potassium t-butoxide, potassium2,2,6,6-tetramethylheptane-3,5-dionate

Na sodium 2,2,6,6-tetramethylheptane-3,5-dionate, sodium ethoxide,sodium t-butoxide

Li lithium 2,2,6,6-tetramethylheptane-3,5-dionate, lithium ethoxidelithium-t-butoxide

Cu Cu(2-ethylhexonate)₂, Cu-nitrate, Cu-acetylacetonate

Pd paladium nitrate (in aqueous ammonium hydroxide solution)(NH₄)₂Pd(NO₂)₂, Pd-acetylacetonate, ammonium hexochloropalladium

Ir H₂IrCl₆ (in 50% ethanol in water solution), Ir-acetylacetonate,Ir-carbonyl

Ag silver nitrate (in water), silver nitrate, silver fluoroacetic acid,silver acetate

Ag-cyclohexanebutyrate, Ag-2-ethylhexanoate

Cd cadmium nitrate (in water), Cd-2-ethylhexanoate

Nb niobium (2-ethylhexanoate)

Mo (NH₄)₆Mo₇O₂₄, Mo(CO)₆, Mo-dioxide bis (acetylacetonate)

Fe Fe(NO₃)₃.9H₂O, Fe-acetylacetonate

Sn SnCl₂.2H₂O, Sn-2-ethylhexanoate, Sn-tetra-n-butyltin, Sn-tetramethyl

In In(NO₃)₃xH₂O, In-acetylacetonate

Bi Bismuth nitrate, Bismuth 2-ethyl hexonate

Ru Ru-acetylacetonate

Zn Zn-2-ethyl hexonate, Zn nitrate, Zn acetate

W W-hexacarbonyl, W-hexafluoride, tungstic acid

In most cases where a mixture of metal precursors and/or metalloidprecursors are deposited, the deposition is generally stoichiometricwith respect to the relative proportions of the metal(s) and/ormetalloid(s) provided by the precursors in the reaction mixtures.However, this relationship is neither precise nor entirely predictable.Nevertheless, this does not present any significant problem in achievinga coating layer or powder of desired composition because the relativeamounts of chemical precursors required to obtain a coating layer orpowder of desired composition can be readily determined without undueexperimentation for any set of coating parameters. Once a ratio ofchemical precursors under a set of coating parameters is determined toobtain a coating or powder of desired composition, the coating can beduplicated with highly predictable results. Thus, if one desired acoating or powder that would contain two metals in a particularpredetermined ratio, one might start out with two chemical precursorscontaining the two metals in the predetermined stoichiometric ratio. Ifdetermined that the two metals were not deposited in the predeterminedratio, adjustments would be made in the relative amounts of the twoprecursor chemicals until the desired ratio of metals in the depositedmaterials was achieved. This empirical determination would then berelied upon for future depositions.

CCVD has the advantages of being able to deposit very thin, uniformlayers which may serve as the dielectric layers of embedded capacitorsand resistors. For embedded resistors, the deposited layers aretypically at least about 40 Å thick. The material can be deposited toany desired thickness; however, for forming resistive material layers byCCVD, thicknesses seldom exceed 50,000 Å (5 microns). Generally filmthicknesses are in the 100-10,000 Å range, most generally in the300-5000 Å range. Because the thinner the layer, the higher theresistance and the less material, e.g., platinum used, the ability todeposit very thin films is an advantageous feature of the CCVD process.The thinness of the coating also facilitates rapid etching in processesby which discrete resistors are formed.

Examples of coatings produced by CCVD include silicon dioxide coatingsproduced from a solution of tetraethoxysilane [Si(OC₂H₅)₄] inisopropanol and propane; platinum coatings produced from a solution ofplatinum-acetylacetonate [Pt(CH₃COCHCOCH₃)₂] in toluene and methanol;and nickel-doped LaCrO₃ coatings produced from solutions of lanthanumnitrate in ethanol, chromium nitrate in ethanol and nickel nitrate inethanol.

The electrical resistance of a resistor is determined by the resistivityof the material, as well as the length and cross-sectional area of theresistor. While very thin films are desirable from the standpoint ofmaterial efficiency, where power loading (current flow) is high, thickerfilms may be required. For higher power loading requirements wherethicker films are required, the resistivity of the material may need tobe higher, e.g., by using more heavily doped metals as the resistivematerial.

Novel resistive materials can be deposited by CCVD and CACCVD such thatvery small, discrete electrical resistors can be formed by CCVD andCACCVD processes in conjunction with conventional or modified printedcircuit board technology. The novel resistive materials are formed bythe co-deposition by CCVD and CACCVD of conductive materials,particularly metals, such as platinum and nickel, with highly resistive(dielectric) materials, such as silica. It is found that a very smallamount of the highly resistive material, e.g., about 0.1 wt % to about20 wt %, very profoundly reduces the conductive properties of theconducting material. For example, platinum, though an excellentconductor, when co-deposited with between 0.1 and about 5 wt % silica,serves as a resistor, the resistance being a function of the level ofsilica co-deposited. While applicants are not bound by theory, it isbelieved that when a conductor and a minor amount of a non-conductor areco-deposited by CCVD or CACCVD, the non-conductor is deposited generallyhomogeneously throughout the conductor, either as single molecules or asnanoclusters of molecules.

For resistive material which is a mixture of a conductive metal and aminor amount of a dielectric material, to be deposited by CCVD orCACCVD, the metal must be capable of being deposited as a zero valencemetal from an oxygen-containing system if the resistive material is tobe deposited by CCVD or CACCVD. The criteria for deposition in the zerovalence state using a flame is that the metal must have a loweroxidation potential than the lower of the oxidation potential of carbondioxide or water at the deposition temperature. (At room temperatures,water has a lower oxidation potential; at other temperatures carbondioxide has a lower oxidation potential.) Zero valence metals which canbe readily deposited by CCVD are those having oxidation potentials aboutequal to silver or below. Thus, Ag, Au, Pt, and Ir can be deposited bystraight CCVD. Zero valence metals having somewhat higher oxidationpotentials may be deposited by CACCVD which provides a more reducingatmosphere. Ni, Cu, In, Pd, Sn, Fe, Mo, Co and Pb are best deposited byCACCVD. Herein, metals also include alloys that are mixtures of suchzero-valence metals. The preferred dielectric materials being capable ofco-deposition with the zero valence metals are metal oxides or metalloidoxides, such as silica, alumina, chromia, titania, ceria, zinc oxide,zirconia, phosphorous oxide, bismuth oxide, oxides of rare earth metalsin general, and mixtures thereof. Silicon, aluminum, chromium, titanium,cerium, zinc, zirconium, magnesium, bismuth, rare earth metals, andphosphorous each have relatively high oxidation potentials, such that ifany oi the metals mentioned above are codeposited with the suggestedprecursors for electrically resistive material, the metals will depositin zero valence state and the dopant will deposit as the oxide. Thus,even when no flame is used the dielectric needs to have a higheroxidation, phospidation, carbidation, nitrodation, or boridationpotential to form the desited two phases.

Again, for more oxygen-reactive metals and alloys of metals, CACCVD maybe the process of choice. Even if the metal can be deposited as a zerovalence metal by straight CCVD, it may be desirable to provide acontrolled atmosphere, i.e., CACCVD, if the substrate material on whichit is to be deposited is subject to oxidation. For example, copper andnickel substrates are readily oxidized, and it may be desired to depositonto these substrates by CACCVD.

Another type of resistive material which can be deposited as a thinlayer on a substrate by CCVD is “conductive oxides”. In particular,Bi₂Ru₂O₇ and SrRuO₃ are conductive oxides which may be deposited byCCVD. Although these materials are “conductive”, their conductivity isrelatively low when deposited in amorphous state; thus, a thin layer ofsuch mixed oxides can be used to form discrete resistors. Likeconductive metals, such “conductive oxides” may be doped with dielectricmaterials, such as metal or metalloid oxides, to increase theirresistivity. Such mixed oxides may be deposited either as amorphouslayers or as crystalline layers, amorphous layers tending to deposit atlow deposition temperatures and crystalline layers tending to deposit athigher deposition temperatures. For use as resistors, amorphous layersare generally preferred, having higher resistivity than crystallinematerials. Thus, while these materials are classified as “conductiveoxides” in their normal crystalline state, the amorphous oxides, even inun-doped form, may produce good resistance. In some cases it may bedesired to form low resistance, 1-100 Ω, resistors and aconduction-enhancing dopant, such as Pt, Au, Ag, Cu or F, may be added.If doped with dielectric material, e.g., metal or metalloid oxides, toincrease resistivity of the conducting oxides, or conduction-enhancingmaterial to decrease resistivity of the conducting oxides, suchhomogeneously mixed dielectric or conduction-enhancing material isgenerally at levels between about 0.1 wt % and about 20 wt % of theresistive material, preferably at least about 0.5 wt %.

There are a variety of other “conducting materials” which thoughelectrically conducting, have sufficient resistivity to form resistorsin accordance with the present invention. Examples include yttriumbarium copper oxides and La_(1-x)Sr_(x)CoO₃, 0≦x≦1, e.g., x=0.5.Generally, any mixed oxide which has superconducting properties below acritical temperature can serve as electrically resistive material abovesuch critical temperature. Deposition of such a variety of resistivematerials is possible with proper selection of precursors selected fromthose described herein above.

To produce a metal/oxide resistive material film, precursor solution isprovided which contains both the precursor for the metal and theprecursor for the metal or metalloid oxide. For example, to produceplatinum/silica films, the deposition solution contains a platinumprecursor, such as platinum(II)-acetylacetonate ordiphenyl-(1,5-cyclooctadiene) platinum (II) [Pt(COD)] and asilicon-containing precursor, such as tetraethoxysilane. The precursorsare mixed generally according to the ratio of metal and the metal ormetalloid (that will form the oxide) to be deposited as a film, however,exact ratios must be determined empirically for any desired ratio ofmetal to oxide. Accordingly, precursor solutions for forming resistivefilms in accordance with the invention contain a precursor that formsthe metal and a precursor that forms the oxide at a weight ratio ofbetween of between about 100:0.2 to about 100:20.

Similarly, when conducting oxides are deposited to form a resistivematerial layer, precursors of each metal, e.g., Bi and Ru, and Sr andRu, are provided in appropriate ratios so as to provide the correctstoichiometry of the conducting oxides. Again, some experimentation maybe required to provide the precursors in a precise ratio for anyparticular deposition conditions so as to produce the desiredstoichiometry of the mixed oxide. Also, where the conducting oxide is tobe doped with a dielectric metal oxide or metalloid oxide to increasethe resistivity of the material being deposited, or conduction-enhancingmaterial to decrease the resistivity of the material being deposited, anadditional precursor is provided so as to produce minor amounts of themetal oxide or metalloid oxide, e.g., between 0.1 and 20 wt %,preferably at least about 0.5 wt %, of the deposited doped conductingmetal oxide.

Either of the above-mentioned platinum precursors are soluble intoluene. Dissolving the platinum precursors is facilitated bysonification. To a solution of the platinum precursor, it is convenientto add tetraethyloxysilane dissolved in methanol, isopropanol or tolueneto form a precursor solution. The precursor solution can then be furtherdiluted to a desired concentration with propane or other organicsolvents.

Generally, for shipping, storage, and handling, the precursor chemicalsare dissolved in common liquid organic solvents, such as toluene,isopropanol, methanol, xylene, and mixtures thereof, to a concentration(of total precursor chemicals) of between about 0.25 and about 20 wt %,preferably at least about 0.5 wt % and typically up to about 5 wt %.Generally, for shipping and handling it is desired to provideconcentrations in a concentrated form to minimize cost and minimize thequantity of flammable liquids. At the same time, stability, particularlylow temperature stability, e.g., down to −20° C. must be consider, lestan overly concentrated solution precipitate from solution. At the timeof deposition, the precursor solutions are typically further diluted,e.g., in propane, to a concentration (of total precursor chemicals) tobetween about 0.005 and about 1.0 wt %, preferably to between about 0.05and about 1.0 wt %, more preferably no more than about 0.6 wt %.

One of the most important metals which can be deposited in doped orundoped form by CACCVD is nickel. Nickel is inexpensive and can beselectively etched relative to conductive metals, such as copper. Animportant precursor for depositing zero valence nickel by CACCVD isnickel nitrate. Nickel may be deposited from an ammoniacal aqueoussolution of nickel nitrate. However, as described above, it is preferredthat deposition be from a liquid at conditions approachingsupercritical. To this end, advantageous carriers for nickel nitrateinclude liquefied ammonia or liquefied nitrous oxide (N₂O). Nitrousoxide may be liquefied by pressurizing to 700-800 psi. Ammonia may beliquefied by pressurization and/or low temperatures. Whether the carrieris liquefied ammonia or liquefied nitrous oxide, it is foundadvantageous to add a minor portion of water, i.e., up to about 40 wt %,preferably between about 2 to about 20 wt %, (the liquefied ammonia orliquefied nitrous oxide comprising the balance, between about 60 andabout 100 wt %). The water raises the supercritical point of eitherliquefied ammonia or liquefied nitrous oxide. This makes it easier tooperate sufficiently below the supercritical point such that variationsin viscosity and density are not encountered. Also, the addition ofwater reduces the instability of the solutions. (It is to be understood,however, that depositions may, in some cases, be carried out inliquified ammonia or liquefied nitrous oxide without the addition ofwater.) In such nickel deposition solutions, the nickel precursor alongwith any precursor for a nickel dopant are typically present at a lowlevel, i.e., from about 0.001 wt % to about 2.5 wt %. Currentlypreferred dopants for nickel are nickel phosphorous and/or nickelphosphorous oxides, e.g., nickel phosphate. It is believed that whenusing a phosphorus-containing precursor, such as phosphoric acid, themajor dopant species is nickel phosphate. Precursor solutions in whichwater and either liquefied ammonia or N₂O are the carrier co-solventsare advantageous in that no carbon is present which could result indeposition of carbon.

When preparing a precursor solution of nickel nitrate to be carried inliquefied ammonia, the nickel nitrate may be conveniently pre-dissolvedin ammonium hydroxide solution along with precursor for any dopant, andthis solution then admixed with liquefied ammonia.

The resistive materials described herein can be fabricated intoresistors, either as embedded resistors, or as resistors on the surfaceof a printed circuit board within integrated circuits or otherelectronic applications. This is generally accomplished using aphotoresist which is used to form a resist pattern over the layer ofresistive material and using an appropriate etchant to remove theresistive material in areas not covered by the resist. For metal/oxideresistive material layers, the etchant chosen is an etchant for themetal component of the resistive material. Typically such etchants areacids or Lewis acids, e.g., FeCl₃ or CuCl₂ for copper. Nitric acid andother inorganic acids (e.g., sulfuric, hydrochloric, and phosphoric) maybe used to etch nickel, a variety of other metals which may be depositedas well as conductive oxides.

Aqua Regia may be used for etching noble metals, such as platinum. AquaRegia is an extremely corrosive acid mixture which, herein, is usefulfor etching metals, particularly noble metals, such as platinum andgold. Au can also be etched in a potassium iodide/iodine (KI/I₂)solution. Because CCVD uses a flame, thereby tending to produce oxides,only the less reactive metals, i.e., those having low oxygen potentials,are easy to deposit as metals, rather than as oxides. Easiest to depositare the noble metals, such as platinum and gold. While these metals are,of course, costly, CCVD can be advantageous in that it can be used todeposit very thin, but nevertheless uniform, films. Accordingly,deposition by CCVD of thin layers of noble metals is, in many casespractical. Furthermore, as noble metals are non-oxidizing, their use inhigh quality electronic applications may easily be economicallyjustified.

Also, although noble metals are conductors, it is found that indepositing noble metals along with relatively minor amounts of oxides,such as silica or alumina, the deposited material becomes highlyresistive. Accordingly, metals, such as platinum, containing minoramounts, e.g., 0.1%-5% of an oxide, can serve as resistors in printedcircuit board. Such material can be deposited as a layer on a printedcircuit board and then processed by printed circuit board techniques toprovide discrete resistors.

However, noble metals, by their non-reactive nature, are difficult toetch, as is required in many processes for production of printed circuitboards. Aqua regia as an etchant for metals, particularly noble metals,in printed circuit board processes.

Aqua regia is made from two well-know acids: 3 parts concentrated (12M)hydrochloric acid (HCl) and 1 part concentrated (16M) nitric acid HNO₃.Thus, the molar ratio of hydrochloric acid to nitric acid is 9:4,although slight variations from this ratio, i.e., 6:4 to 12:4 would beacceptable for etching purposes in accordance with the invention.Because of its corrosive nature and limited shelf life, Aqua regia isnot sold commercially, but must be prepared on site. To reduce itscorrosiveness, the Aqua regia may be diluted with water up to about a3:1 ratio of water to aqua regia. Dilution with water, of course,increases the etching time, but good etching times of platinum areachievable with a 33% aqua regia solution. Of course, more reactivemetals, such as copper, will be easily etched as well by aqua regia. Onthe other hand, the noble metals, such as platinum, are not etched bymany of the materials suitable for etching copper, such as FeCl₃ orCuCl₂, thereby allowing for a variety of selective etching options informing printed circuit boards.

The speed of etching will depend upon several factors including thestrength of the aqua regia and the temperature. Preferably the aquaregia is prepared fresh. Typically, aqua regia etching is conducted inthe 55-60° range, although this may be varied depending upon theapplication.

The following discussion of formation of discrete resistors assumes theuse of a platinum-based resistive material because platinum/silica isthe currently preferred CCVD-deposited resistive material. However, itis to be understood that the other resistive materials, including bothmetal/oxide and conducting oxide films as described above, can besubstituted. Likewise, in techniques described hereinafter in whichcopper and platinum-based resistive layers are selectively etched, it isto be understood that there are selective etchants available for avariety of conductor/resistive material combinations in accordance withthe invention.

In its simplest form, a resistor 400 in accordance with the invention ismerely a patch or strip 401 (FIGS. 4c and 4 d) of the thin layer resistmaterial on an insulating substrate 402 with means, such as a contactingcopper patch 403 at each ends to provide for electrical connection ofthe resistor to electronic circuitry. The substrate 402 might be aflexible sheet, such as a polyimide sheet, a rigid epoxy/fiberglassboard, or even liquid crystal sheet material. Suitable substratesdesired for many applications are films of organic polymers, such aspolyimide, having thicknesses of about 10 microns or less. Afteroptimizing deposition parameters, it was found, herein, that CCVD canapply resistive material layers to insulating substrates, such aspolyimide, without burning or deforming the substrates. Directdeposition of the resistive material layer on an insulating substrategenerally provides good adhesion of the resistive material layer to theinsulating substrate. Usually, such adhesion is better than prior arttechniques which use an adhesive to bind a resistive material to asubstrate. To form a discrete resistor 400, a thin layer of resistmaterial 401(4 a) is deposited by CCVD on an insulating substrate 402 toform the structure of FIG. 4a. A chemical-resistant photoresist, such asthat sold by Morton Electronic Materials as Laminar 5038 which isresistive to aqua regia (in the case of platinum etching), is applied tothe exposed surface of the resist material and patterned by conventionalphotoimaging techniques. Generally, a resist which will withstand veryhighly acidic conditions, such as gold-plating conditions, will besuitable as a resist for etching with aqua regia. The exposed portionsof the resist material layer are then etched away, by aqua regia in thecase of noble metal-based resist materials, leaving the patches orstrips 401(4 b) of resist material so as to form the structure of FIG.4b. Copper connecting patches 403 may then be applied to the ends of thestrips 401 to form the resistor 400 of FIG. 4c.

Preferably, however, in reference to FIGS. 5a-5 c, both the thin layerresist material patches 401 and the electrical connection conductivepatches 403 are formed by photoimaging techniques. Shown in FIG. 5a is athree-layer structure 409 which comprises an insulating substrate 402, alayer of resist material 401(5 a), e.g., Pt/silica, formed in accordancewith the invention by CCVD and a conductive layer 403(5 a), e.g.,copper, formed by CCVD or another technique, such as electrolyticplating.

The structure 409 of FIG. 5a might be patterned in one of two two-stepprocedures by photoimaging technology. In one procedure (with referenceto FIG. 5b), the conductive material layer 403(5 a) would be coveredwith a resist, the resist patterned by photoimaging techniques, and, inthe exposed areas of the resist, both the conductive material layer andthe underlying resistive material layer be etched away, e.g., with aquaregia to give the structure of FIG. 5b having a patterned resistivematerial patch (401(5 b)) and a patterned conductive material patch(403(5 b)). Next, a second resist would be applied, photoimaged, anddeveloped. This time, only the exposed portions of the conductivematerial patch 403(5 b) would be etched away by etchant which wouldselectively etch the conductive layer, but not the resistive materialpatch, i.e., FeCl₃ or CuCl₂ in the case of Cu as the conductive materiallayer and Pt/silica as the electrically resistive material, therebyproducing the resistor structure 400 of FIG. 4c. In an alternateprocedure (with reference to FIG.5c), a patterned resist layer would beformed, exposed portions of the conductive material layer 403(5 a)etched away, e.g., with FeCl₃, a further patterned resist layer formed,and then the exposed areas of the resistive material layer (401(5 b))etched away with aqua regia so as to form the electrical contacts 403and give the resistor structure 400 of FIG. 4c. By either procedure,discrete thin layer resistors 400 are formed by conventionalphotoimaging techniques common to printed circuitry formation.

Still another way of forming discrete resistors is to start with atwo-layer structure such as that shown in FIG. 4a having a layer ofresistive material, e.g., Pt/silica, on an insulating substrate. Using aphotoresist process, discrete patches or strips of the resistivematerial are formed on the substrate, giving a structure such as thatshown in FIG. 4b. Next, a layer of conductive material, e.g., copper, isformed on the resistive patches or strips, e.g., by electrolyticplating, giving a structure such as is shown in FIG. 5b. A furtherphotoresist is applied and imaged, and exposed portions of theconductive material are then etched away so as to leave the conductiveelectrical connection patches 403 and provide a resistor structure 400such as is shown in FIG. 4c.

While the resistor 400 of FIG. 4c could be at the surface of a printedcircuit board device, the resistors will, in most cases, be embeddedwithin a multi-layer printed circuit board as shown in FIG. 6 where theresistor 400, which was formed on an insulating substrate 402, such aspolyimide, is embedded within additional insulating material layers 420,such as epoxy/fiberglass prepeg material.

Illustrated in FIGS. 9a-g are cross-sectional views representing acircuitization process which begins with a conductive foil 900, such asa copper foil, to which a layer of electrically resistive material 905has been deposited by CCVD or CACCVD, this two-layer structure beingrepresented in FIG. 9a. Copper foil useful in this process is typicallybetween about 3 and about 50 microns thick.

Photoresist layers 910 and 915 are then applied to both sides of thetwo-layer structure. The photoresist 910 covering the resistive materiallayer 905 is exposed to patterned actinic radiation while thephotoresist 915 covering the conductive foil 900 is blanket-exposed toactinic radiation. The photoresists are then developed, giving thestructure of FIG. 9b with a patterned photoresist layer covering theresistive material layer 905 and the blanket-exposed photoresist layer915 protecting the conductive foil.

As shown in FIG. 9c, the resistive material layer 905 is thenselectively etched from areas where the photoresist 910 had beenremoved. Subsequently, the remaining photoresist 910, 915 is stripped.

Following this, as shown in FIG. 9d, an organic laminate 920 is appliedto the resistive material side of the structure. The laminate protectsthe now-patterned resistive material layer 905 from further processingand subsequently supports patches of the resistive material layer 905when portions of the conductive foil is subsequently removed from theother side of the resistive material layer.

Next, a photoresist layer 925 is applied to the conductive foil 900.This is imaged with patterned actinic radiation and developed, givingthe structure shown in FIG. 9e. Following this, the conductive foil 900is etched with an etchant which selectively etches the conductive foil900 but which does not etch the resistive material layer 905, leavingthe structure shown in FIG. 9f. Stripping of the photoresist 925 leavesthe resisttor structure shown in FIG. 9g. This structure maysubsequently be embedded in dielectric material (not shown).

As a variation of this process, it should be noted that if an etchant isused which selectively etches the electrically resistive material layer905 but does not etch or only partially etches the conductive foil 900,the use of resist layer 915 (FIGS. 9b and 9 c) is not necessary.

When referring herein to “etching”, the term is used to donate not onlythe common usage in this art where a strong chemical dissolves thematerial of one of the layers, e.g., nitric acid dissolves nickel, butalso physical removal, such as laser removal and removal by lack ofadhesion. In this regard, and in accordance with an aspect of thisinvention, it is believed that resistive materials, such as doped nickeland doped platinum, deposited by CCVD or CACCVD are porous. It isbelieved that this porosity permits liquid etchants to diffuse throughthe electrically resistive material layer and, in a physical process,destroy the adhesion between the resistive material layer and theunderlying layer.

For example, in reference to FIGS. 9b and 9 c, if the conductive foillayer 900 is copper and the resistive material layer 905 is dopedplatinum, e.g., Pt/silica, or doped nickel, e.g., Ni/PO₄, cupricchloride could be used to remove exposed portions of the resistivematerial layer. The cupric chloride does not dissolve either Pt or Ni,but the porosity of the resistive material layer allows the cupricchloride to reach the underlying copper. A small portion of the copperdissolves and the exposed portions of the electrically resistive layer905 by physical ablation. This physical ablation occurs before thecupric chloride etches the underlying copper layer 900 to anysignificant extent.

By the same token, the porosity of the resistive materials deposited inaccordance with the invention may be removed by ablative etching. Forexample, a platinum layer on a polyimide substrate may be etched usingetchants, such as those described above with respect to removing aresistive layer from a conductive copper substrate, particularlyinorganic acids such as hydrochloric acid, sulfuric acid and acidiccupric chloride. Thus, in processes, such as heretofore described usingcommon photoresist techniques, discrete resistors may be formed byetching thin films of resistive materials on insulating substrates, suchas polyimide films.

If copper is the conductive material layer 900, it is sometimesadvantageous to use copper foil that has been oxidized; oxidized copperfoil is commercially available. An advantage of an oxidized copper foilis that a dilute HCl solution, e.g., ½%, dissolves copper oxide withoutdissolving zero valence copper. Thus, if the electrically resistivematerial layer is porous, such that the dilute HCl solution diffusestherethrough, HCl can be used for ablative etching. Dissolving thesurface copper oxide destroys the adhesion between the copper foil andthe electrically resistive material layer. As noted above with respectto the process shown in FIGS. 9a-9 g, the use of such an etchant whichdoes not attack the foil dispenses with the need for protectivephotoresist layer 915 (FIGS. 9b and 9 c).

To minimize processing steps, the photoresists applied can be embedablein materials, such as Morton International's permanent etch resist. Thenboth sides can be processed simultaneously if the etchant does not oronly partially etches the conductor. In particular, only the resistormaterial side photoresist needs to be embeddable and the conductor sidecan be removed as a final processing step. Alternatively, thephotoresists used on the conductor material side can be selected suchthat it is not removed with a specific stripper used to remove theresistor material side photoresist. Embedable photoresist may decreasetolerance losses due to particular undercutting of resistor materialwhich under cut material will ablate once the photoresist is removed.

It can be demonstrated that when using porous electrically resistivematerial layers, such as doped platinum and doped silica, with certainetchants, the etching process is a physical ablation process. This isbecause flakes of the electrically resistive material layer are found inthe etchant bath. Because of this, separation of the ablated resistivematerial can be separated from the etchant bath by physical means, suchas filtration, settling, centrifugation, etc. This is particularlyconvenient for recovering expensive material, such as platinum.

To be practically removable by an ablative technique, the resistivematerial layer must generally be sufficiently porous to an etchant whichdoes not dissolve the electrically resistive material but sufficientlyattacks the surface of the underlying material such as to result in lossof interfacial adhesion and ablation of the electrically conductivematerial within about 2 to 5 minutes. At the same time, such etchantmust not substantially attack the underlying material, e.g., copperfoil, during the etching period as such would cause excessiveundercutting or loss of mechanical strength (i.e., reducehandleability).

Thus, with respect to the structure described above, there is asillustrated in FIG. 10a a conductive layer 1000, e.g., copper; anintermediate etchable layer 1002, e.g., copper oxide; and a porous layer1004 of resistive material through which an etchant may seep anddissolve the intermediate layer without significantly degrading theconductive layer. With respect to FIG. 10b a patterned resistive layer1006 is formed by light-exposure and development; then, with respect toFIG. 10c, a patterned resistive layer is formed by ablative etching ofthe resistive layer 1004 by exposure to an etchant that seeps throughthe porous resistive layer and attacks the intermediate layer 1002,whereby the overlying resistive layer may be mechanically ablated.

Though copper oxide is a suitable intermediate layer 1002 from thestandpoint of being selectively etchable relative to the underlyingcopper conductive layer 1000, it is not the preferred material for anintermediate layer 1002. It is found that when a resistive material,such as silica-doped platinum, is deposited directly onto either copperor copper oxide, there is a tendency for the copper and/or copper oxideto interact with the resistive material such that the resistivity of theresistive material may be unpredictable. Preferably, therefore, beforeapplying the resistive material by CCVD or CACCVD, an intermediate layer1002 is coated onto the conductive foil layer 1000, the material beingsuch that it does not allow the conductive material from the foil layer1000 to diffuse into the resistive material layer 1004.

The requirements of material for the intermediate layer 1002 must besuch that the material be etchable by an etchant which degrades theintermediate layer sufficiently to ablate the resistive material layer1004. It is preferred that the etchant be such that it minimallydegrades or does not degrade the conductive layer 1000. It may be, forexample, that there exists a chemical which etches the intermediatelayer but which does not react with the conductive layer 1000. However,even if a chemical degrades both the material of which the intermediatelayer 1002 is formed and the material of which the conductive layer 1000is formed, it is generally still possible to use such an etchant bycontrolling the etching conditions, including time, such that theintermediate layer 1002 is degraded without substantial degradation ofthe conductive layer 1000. For example, if the conductive layer 1000 iscopper and the intermediate layer 1002 is nickel, cupric chloride, whichdegrades both nickel and copper is a suitable etchant providing that theetching conditions are controlled such that the very thin nickel layeris substantially degraded but the relatively much thicker copper layeris not significantly degraded. Furthermore, the material of theintermediate layer 1002 must permit good electrical contact to bemaintained between the conductive layer 1000 and the resistive materiallayer.

One choice of material for an intermediate layer 1002 is a metal, suchas nickel, which prevents interaction between the copper and theresistive material layer 1004 by providing a barrier between theconductive layer, e.g., copper. Nickel may be deposited on copper, forexample by electroplating. Typically, a nickel intermediate layer wouldbe between about 2 and about 6 microns, although the thickness is notconsidered to be particularly critical.

Another choice of an intermediate layer 1002 material is a ceramic, suchas silica or another metal or metalloid oxide. Such an intermediatelayer may be deposited by CCVD as described above prior to depositingthe layer 1004 of resistive material. While most ceramic materials, suchas silica, are electrically insulating (dielectric), if deposited as asufficiently thin, layer, e.g., averaging between about 15 and about 50nanometers, a dielectric material still acts as an intermediate barrierlayer 1002 without significantly disrupting electrical contact betweenthe conductive layer 1000 and the resistive layer 1004. (When discussingintermediate layer thicknesses, what is being discussed is the mean oraverage thickness, as the thickness typically varies from location tolocation depending upon factors such as the roughness of the substrateand the deposition conditions.) The net effect is an etchable,electrically leaky intermediate layer which is an effectivecompositional buffer.

Suitable etchants for silica, if used as an intermediate layer, includeammonium hydrogen difluoride, fluoroboric acid and mixtures thereof. Oneparticularly suitable etchant for silica, if used as the intermediatelayer is an aqueous solution of 1.7 wt % ammonium hydrogen difluoride,and 1.05 wt % fluboric acid. Other materials can be added to a mixtureof these two components.

In the case of silica, a sufficiently nano-porous or defective coatingto enable nano spots of direct contact between the resistor andconductor is desired. Such contacts can be 1-100 nm in size and form on0.05% to 10% of the area, thus allowing resistor feature sizes even downto micron scale resolution while still providing excellent electricalcommunication. This still sufficiently reduces material interaction.Alternatively, poor insulator, semiconducting or conductive compositeceramic or polymer materials could be used, in which case these could bethicker. Also, in this regard, the rougher the substrate surface, thethicker the intermediate layer may be because a rougher substratesurface tends to produce a more porous intermediate layer coating. Thatis, it is believed that the rougher the surface of the substrates, thegreater the number of pinholes produced in the intermediate coating,pinholes through which electrical contact may be maintainted.

Other oxides which may be used as an intermediate layer include zincoxide, strontium oxide, and tungsten oxide. Each of these oxides can bedeposited by CCVD using zinc, strontium and tungsten precursorsdescribed above. Each of these oxides can be applied to coppersubstrates by CCVD at sufficiently low temperatures that the copper isnot oxidized. Each of these oxides can be applied at relatively lowcost.

Zinc oxide is an especially promising intermediate layer material inthat it is a semiconductor of electricity. Therefore, it provides betterelectrical continuity between the conductive metal, e.g., copper, andthe resistor. Zinc oxide (as well as other oxides) can be doped toincrease conductivity. Also, Zinc oxide is etchable with hydrochloricacid.

Strontium oxide and tungsten oxide are etchable with strong bases, suchas KOH.

The invention will now be described in greater detail by way of specificexamples.

EXAMPLE 1

A layer of Pt/SiO₂ resistive material was deposited by CCVD on polyimideusing deposition conditions as follows:

Solution preparation: 1.23 g Pt(COD) 250 ml toluene 0.43 g TEOS (1.5 wt% Si in toluene) 150 g propane Deposition conditions: Solution flow: 3ml/min Deposition time: ˜18 min for 5″ × 6″ substrate # of passes: 6Deposition temp. 500° C. Variac 3.0 A Tip Oxygen flow: ˜2900 ml/min

The sample described by the deposition conditions above yielded aresistance value of ˜17 ohms per square.

This is an example of a 65% concentrated solution with 2.5 wt % SiO₂.The variables that can be changed include the amount of Pt(COD) and TEOSadded proportionally to reach concentrations to 100% solution (e.g.,1.89 g Pt(COD) and 0.65 g TEOS (1.5 wt % Si)) and the amount of TEOSthat can be added to change the resulting weight % SiO₂ (typically 0.5-5wt % are used for this project).

EXAMPLE 2

In some cases, there will be the need to deposit certain materials ontooxidation sensitive substrates without oxidizing the substrate. This canbe done using the CACCVD technique, and the deposition of the dielectriccompound SrTiO₃ onto Ni is one example. This deposition uses thetraditional CCVD nozzle which is placed in a jacket that can supplyinert or reducing gases around the flame. This jacketed nozzle is thenhoused in a quartz tube to prevent air from reaching the substrateduring the deposition as shown in FIG. 8. For this CACCVD flame, acombustible solution flows through the needle as in the CCVD process,oxygen flows through the tip, and hydrogen flows through the pilottubes. High flows of inert (such as argon or nitrogen) or reducing gases(such as 90-99.5% argon/10-0.5% hydrogen) flow through the jacket aroundthe flame. For very small samples, a side arm purged with an inert orreducing gas is part of the quartz tube to allow a heated sample to coolin a controlled atmosphere after the deposition and therefore preventoxidation at this point. This process has allowed SrTiO₃ to be depositedonto Ni without forming NiO or without depositing carbon as far as EDXand XRD analysis have indicated. Early experiments have shown thatsolvents with a low carbon deposition potential such as methanol arebetter to use than toluene. Carbon was deposited onto the substrate whentoluene was used. The ideal processing parameters to date are givenbelow.

Solution preparation: 0.82 g Sr 2-ethylhexonate (1.5 wt % Sr in toluene)0.73 g Ti -di-i-acac (0.94 wt % Ti in toluene) 17 ml methanol l00 gpropane Deposition conditions: Solution flow: 2 ml/min Deposition time:15 min. (has varied from 10-15 min) Deposition temp. ˜950° C. (hasvaried from 800-1050° C.) Variac 1.9 A (has varied from 1.9-2.25 A)Pilot Hydrogen flow: ˜1926 ml/min (has been as low as 550 ml/m) TipOxygen flow: ˜1300 ml/min (has varied from 500-2322 ml/m) Reducing gasmix: 0.5-10% hydrogen/balance argon Reducing gas flow: 58 l/min

EXAMPLE 3

A phosphate-doped nickel film was deposited onto 200TAB-E, polyimidesubstrates utilizing a solution of 0.760 g Ni(NO₃) H₂O and 0.30 g H₃PO₄in 400 ml 6M NH₄OH using the described in FIG. 7. The solution wasflowed through a 22 ga. stainless steel needle with a 22 μm ID (0.006″OD) fused silica capillary insert (3 mm long) at the tip 722, at a flowrate of 0.50 sccm. Hydrogen gas was passed through the surrounding tube738 at a rate of 1.20 lpm. Hydrogen was passed through the tube 744surrounding that at a rate of 756 sccm. Oxygen gas was passed throughthe tube 752 surrounding that at a rate of 1.40 lpm. Argon gas waspassed through the outer tube 768 at a rate of 28.1 lpm. All flows werestarted prior to manual ignition of the flame. Generally, the argon flowhad to be reduced for the flame front to ignite the inner nozzle. Theargon flow was then returned to its initial setting. Once lit, no pilotor ignition source was required to maintain combustion. The gastemperature at approximately 1 mm above the deposition point was 500° C.The substrate was rastered at 2 mm from the nozzle collar at 20″/minwith 0.0625″ steppings traversing an area of 3.5″ by 3.5″ once withhorizontal sweeps. The total time required for this rastering motion was12 minutes.

The linear resistance of the deposited phosphate-doped nickel layer was115 Ω/in.

For comparison, the deposition was repeated with a solution which didnot contain the phosphoric acid. The resistance of the nickel layer was5 Ω/in.

EXAMPLE 4

Bi₂Ru₂O₇ was deposited using the following chemicals and processparameters:

Precursor solution:

0.0254 wt % of Bi in 2-ethylhexanoate+0.0086 wt % of Ru inacetylacetonate+1.8026 wt % methanol+15.0724 wt % of toluene+83.0910 wt% of propane.

Parameters:

Flow rate of precursor solution: 3 ml/min.

Tip oxygen flow rate: 4 l/min.

Variac: 2.30A.

No back cooling.

Deposition temperature: 250-650° C.

Amorphous Bi₂Ru₂O₇ was coated at 400° C. gas temperature and theelectrical resistivity was less than 7200 μΩ•cm; this is the best modeto date. Propane and toluene were used as solvents. To prepareconcentrated or diluted solution for deposition, toluene ranging from 1to 35 wt % can be used. Propane in the range of 99 to 65 wt % can alsobe utilized. By changing the solvent weight percentages, theconcentrations of solutes (Bi-2-ethylhexanoate and Ru-acetylacetonate)can be adjusted accordingly. Flow rate of precursor solution can rangefrom 1-5 ml/min.

EXAMPLE 5

SrRuO₃ was deposited using the following chemicals and processparameters:

Precursor Solution:

0.0078 wt % of Sr in 2-ethylhexanoate+0.0090 wt % of Ru inacetylacetonate+12.7920 wt % of toluene+87.1912 wt % of propane.

Parameters:

Flow rate of precursor solution: 3 ml/min.

Tip oxygen flow rate: 4 l/min.

Variac: 2.75A.

No back cooling.

Deposition temperature: 300-650° C.

Amorphous SrRuO₃ was coated at 400° C. gas temperature and theelectrical resistivity was less than 5400 μΩ•cm; this is the best modeto date. Propane and toluene were used as solvents. To prepareconcentrated or diluted solution for deposition, toluene ranging from 1to 35 wt % can be used. Propane in the range of 99 to 65 wt % can alsobe utilized. By changing the solvent weight percentages, theconcentrations of solutes (Sr-2-ethylhexanoate and Ru-acetylacetonate)can be adjusted accordingly. Flow rate of precursor solution can rangefrom 1-5 ml/min.

EXAMPLE 6

Method One: Formation of Singular Discreet Resistors

On a polyimide sheet, 25 microns thick, there was deposited a 200nanometer thick platinum/silica layer (Pt:SiO₂, 97.5:2.5) according tothe method of (Example 1). To the platinum layer was laminated a photoresist, Laminar 5000 Series, sold by Morton International ElectronicsMaterials. The resist layer was covered with a photo tool, and theuncovered portions of the resist layer exposed with 70 millijoules of UVlight. The unexposed resist was then removed by developing in a 1%sodium carbonate monohydrate solution at 80° F. using a conveyorizedspray developer at about 25 psi with a residence time adjusted so thatthe breakpoint occurred at 40% to 50% of the chamber length, followed byseveral spray rinses using tap water and deionized water.

Next, the sheet was exposed to a 50% solution of aqua regia (500 mlH₂O+125 ml HNO₃+375 ml HC.) solution at 50° C. for a sufficient time toremove all of the Pt/SiO₂ material in those regions from which theresist had been removed thus forming the discreet resistors.

EXAMPLE 7

Method Two: Formation of Singular Discreet Resistors with CopperConnecting Circuits

On a polyimide sheet, 25 microns thick, there was deposited a 200nanometer thick platinum/silica layer (Pt:SiO₂, 97.5:2.5) according tothe method of (Example 1). Copper was then plated directly to thesurface of the Pt/SiO₂ layer to a thickness of 12 microns using acommercial vendor supplied acid copper plating bath using standardvendor supplied plating parameters. To the plated copper layer waslaminated a photo resist, Laminar 5000 Series, sold by MortonInternational Electronics Materials. The resist layer was covered with aphoto tool, and the uncovered portions of the resist layer exposed with70 millijoules of UV light. The unexposed resist was then removed bydeveloping in a 1% sodium carbonate monohydrate solution at 80° F. usinga conveyorized spray developer at about 25 psi with a residence timeadjusted so that the breakpoint occurred at 40% to 50% of the chamberlength, followed by several spray rinses using tap water and deionizedwater.

Next, the sheet was exposed to a 50% solution of aqua regia (500 ml H₂O+125 ml HNO₃+375 ml HCl.) solution at 50° C. for a sufficient time toremove all of the plated copper and the Pt/SiO₂ material in thoseregions from which the resist had been removed, thus forming theelectronic circuit pattern. The photo resist was removed in a 3%solution at 130° F. of sodium hydroxide using a conveyorized sprayresist stripper at about 25 psi with a residence time adjusted so thatthe breakpoint occurred at 40% to 50% of the chamber length, followed byseveral spray rinses using tap water and deionized water.

To the circuitized electronic pattern was laminated a photo resist,Laminar 5000 Series, sold by Morton International Electronics Materials.The resist layer was covered with a photo tool, and the uncoveredportions (all areas other than the area of the discreet resistors) ofthe resist layer exposed with 70 millijoules of UV light. The unexposedresist was removed by developing in a 1% sodium carbonate monohydratesolution at 80° C. using a conveyorized spray developer at about 25 psiwith a residence time adjusted so that the breakpoint occurred at 40% to50% of the chamber length, followed by several spray rinses using tapwater and deionized water. The exposed copper area was then etched in acupric chloride commercial bendor supplied etchant to remove only thecopper leaving the Pt/SiO₂ exposed and unetched, thus forming theresistors connected at each end by copper circuit traces. The photoresist is removed in a 3% solution at 130° F. of sodium hydroxide usinga conveyorized spray resist stripper at about 25 psi with a residencetime adjusted so that the breakpoint occurred at 40% to 50% of thechamber length, followed by several spray rinses using tap water anddeionized water.

EXAMPLE 8

Method Three: Formation of Singular Discreet Resistors with CopperConnecting Circuits

On a polyimide sheet, 25 microns thick, there was deposited a 200nanometer thick platinum/silica layer (Pt:SiO₂, 97.5:2.5) according tothe method of (Example 1). Copper was then plated directly to thesurface of the Pt/SiO₂ layer to a thickness of 12 microns using acommercial vendor supplied acid copper plating bath and platingparameters. To the plated copper layer was laminated a photo resist,Laminar 5000 Series, sold by Morton International Electronics Materials.The resist layer was covered with a phototool, and the uncoveredportions of the resist exposed with 70 millijoules of UV light. Theunexposed resist was then removed by developing in a 1% sodium carbonatemonohydrate solution at 80° F. using a conveyorized spray developer atabout 25 psi with a residence time adjusted so that the breakpointoccurred at 40% to 50% of the chamber length, followed by several sprayrinses using tap water and deionized water.

The copper was then etched in a cupric chloride vendor supplied etchantexposing the Pt/SiO₂.

The resist was stripped and a new layer of photo resist (Laminar 5000Series) applied using an industry standard vacuum lamination process. Asecond photo mask having line widths two mils wider than the originalpatter was used to expose the second pattern using identical exposureparameters as used in the original resist expose operation.

Next, the sheet was exposed to a 50% solution of aqua regia (500 mlH₂O+125 ml HNO₃+375 ml HCl) solution at 50° C. for a sufficient time toremove all of the exposed Pt/SiO₂ material in those regions from whichthe resist had been removed thus forming the electronic circuit pattern.The photo resist was removed in a 3% solution at 130° F. of sodiumhydroxide using a conveyorized spray resist stripper at about 25 psiwith a residence time adjusted so that the breakpoint occurred at 40% to50% of the chamber length, followed by several spray rinses using tapwater and deionized water.

To the sheet was laminated a third new photo resist layer, Laminar 5000Series, sold by Morton International Electronics Materials. The resistlayer was covered with a phototool, and the uncovered portions (allareas other than the area of the discreet resistors), exposed with 70milijoules of UV light. The unexposed resist was then removed bydeveloping in a 1% sodium carbonate monohydrate solution at 80° C. usinga conveyorized spray developer at about 25 psi with a residence timeadjusted so that the breakpoint occurred at 40% to 50% of the chamberlength, followed by several spray rinses using tap water and deionizedwater. The exposed copper area was then etched in a commercial vendorsupplied cupric chloride etchant to remove only the copper leaving thePt/SiO₂ exposed and unetched, thus forming the resistors connected ateach end by copper circuit traces. The photo resist is again removed ina 3% solution at 130° F. of sodium hydroxide using a conveyorized sprayresist stripper at about 25 psi with a residence time adjusted so thatthe breakpoint occurred at 40% to 50% of the chamber length, followed byseveral spray rinses using tap water and deionized water.

EXAMPLE 9 Resistor with Silica Barrier Layer

This is an example of how to produce an embedded resistor using a SiO₂barrier.

Starting with copper foil of the desired finished circuit tracethickness, a barrier layer of SiO₂ approx. 20 to 50 nanometers thick isdeposited on the copper foil by CCVD deposition. This can beaccomplished either by depositing on a single sheet of foil or by usinga roll (reel to reel) process.

Following the barrier layer deposition process the resistor material(e.g. Pt metal doped with 2.5% SiO₂) is deposited to a thickness ofapprox. 100 to 150 nano-meter thickness using the CCVD process. Thequality of the deposited material is tested at this point for thickness,composition and bulk resistivity.

An actual resistor material sample consisted of an amorphous silicacoating as the barrier layer and an overlaying resistor layer ofplatinum silica composite. The substrate was copper foil of 24″×30″ sizeand with coated area of 18″×24″.

The solution of resistor precursors contained 0.512 wt %diphenyl(1,5-cyclooctadiene) platinum (II), 0.028 wt % oftetraethoxysilane, 58.62 wt % of toluene and 40.69 wt % propane. Thesolution of silica precursor contained 0.87 wt % tetraethoxysilane, 8.16wt % isopropyl alcohol and 90.96 wt % propane. Depositions of theresistor coating with silica barrier layer also used the solutions ofPt(SiO₂) precursors at lower concentrations, such as 80%, 75%, 65% and50% of the above concentration.

Deposition was performed using the four-nozzle CCVD system at 650° C.for the first pass of silica, at 750° C. for the second pass of silicaand at 700° C. for the overlaying three passes of Pt(SiO₂) resistorcoating.

To remove the unwanted resistor material from the package a photoimageable etch resist (e.g. Laminar 5000) is coated over the resistormaterial. The photo resist material is exposed using standard photoprocessing techniques (e.g. UV light exposure through a photo mask) andthe unpolymerized photo resist is removed using the appropriate solvent(e.g. 2% sodium carbonate solution at 80° C.) uncovering the resistormaterial which is to be removed in the subsequent ablative etch process.The package is then processed through a spray etch machine where asolution of glass etchant (e.g. 1.7 wt % ammonium hydrogen difluorideand 1.05 wt % fluboric acid in water) for sufficient time to chemicallyattack the SiO₂ barrier layer and ablate the unwanted resistor material.The principle of this process is that the etchant penetrates micro poresin the resistor material attacking the underlaying SiO₂ layer. As theSiO₂ layer is solublized by the glass etchant, the resistor materialloses adhesion and due to the thinness is broken up into small piecesand is carried away as a solid in the sprayed etchant material. Exposureto the etchant is limited to a period of time sufficient to remove theresistor material but not long enough (approx. 15 to 60 seconds) tocause undercutting of the desired material covered by the photo resist.

The resistor material is then transferred to a layer of standard epoxylaminate material using commercial lamination processes by placing onesheet of 76289 prepreg over the resistor material side of the etchedresistor package followed by an organic release sheet. This package wasplaced in a standard PWB laminating press and cured using standardlaminating conditions. After lamination the release sheet is peeled fromthe laminate package and the copper is removed to expose the resistorsand to form the connection circuit traces. The copper removal process isaccomplished using standard photo processing techniques and etching withcupric chloride. Resistors are formed by removing the copper from overthe surface while leaving the copper to connect to both ends of theresistor.

EXAMPLE 10 Resistor with Nickel Barrier Layer

An example of how one would produce a buried (embedded) resistor using anickel barrier is as follows.

Starting with copper foil of the desired finished circuit tracethickness, a barrier layer of nickel metal approx. 2 to 5 microns isdeposited on the copper foil either by electro plating or by CCVDdeposition. This can be accomplished either by depositing on a singlesheet of foil or by using a roll (reel to reel) process.

Following the barrier layer deposition process the resistor material(e.g. Pt metal doped with 2.5% SiO₂) is deposited to a thickness ofapprox. 100 to 150 nano-meter thickness using the CCVD process. Thequality of the deposited material is tested at this point for thickness,composition and bulk resistivity.

Actual resistor samples, with a Nickel barrier layer were processed .The samples consisted of three 18″×24″ copper sheets which had beenelectro nickel plated using a commercial nickel plating bath. Threethicknesses of nickel were deposited to a thickness of approx. 3.5, 7.0.and 10.5 microns. The substrate was commercial copper foil used in theproduction of standard PWB (Printed Wiring Boards).

The resistor material was deposited using a solution of resistorprecursors contains 0.512 wt % diphenyl(1,5-cyclooctadiene) platinum(II), 0.028 wt % of tetraethoxysilane, 58.62 wt % of toluene and 40.69wt % propane. Depositions of the resistor coating with nickel barrierlayer also used the solutions of Pt(SiO₂) precursors at lowerconcentrations, such as 80%, 75%, 65% and 50% of the aboveconcentration.

Deposition was performed using the four-nozzle CCVD system at 700° C.for the overlaying three passes of Pt(SiO₂) resistor coating.

To remove the unwanted resistor material from the package a photoimageable etch resist (e.g. Laminar 5000) is coated on both sides of theresistor material package (if a selective etchant material, one whichwill etch nickel and not the copper is used to ablatively etch theresistor, only the resistor material side has to be coated with thephoto resist material). The photo resist material is exposed usingstandard photo processing techniques (e.g. UV light exposure through aphoto mask) and the unpolymerized photo resist is removed using theappropriate solvent (e.g. 2% sodium carbonate solution at 80° C.)uncovering the resistor material which is to be removed in thesubsequent ablative etch process. The package is then processed througha spray etch machine where commercial cupric chloride etch solution issprayed on the part causing the ablative etch of the resistor material.The principle of this processes that the etchant penetrates micro poresin the resistor material attacking the underlaying nickel layer. As thenickel layer is solublized by the cupric chloride, the resistor materialloses adhesion, and due to the thinness is broken up into small piecesand is carried away as a solid in the sprayed etchant material. Exposureto the etchant is limited to a period of time sufficient to remove theresistor material but not long enough (approx. 15 to 60 seconds) to etchthrough the copper foil carrier.

The resistor material is then transferred to a layer of standard epoxylaminate material using commercial lamination processes by placing onesheet of 76289 prepreg over the resistor material side of the etchedresistor package followed by an organic release sheet. This package wasplaced in a standard PWB laminating press and cured using standardlaminating conditions. After laminations the release sheet is peeledfrom the laminate package and the copper is removed to expose theresistors and to form the connection circuit traces. The copper removalprocess is accomplished using standard photo processing techniques andetching with cupric chloride. Resistors are formed by removing thecopper from over the surface while leaving the copper to connect to bothends of the resistor.

EXAMPLE 11 Strontium Oxide Barrier Layer Deposition

Strontium oxide coatings were deposited onto Cu foil using the CCVDprocess. During the deposition the solution flow rate, oxygen flow rateand cooling air flow rate were kept constant. The solution of thestrontium oxide precursor contained 0.71 wt % strontium2-ethylhexanoate, 12.75 wt % toluene, and 86.54 wt % propane. The flowrate for the solution was 3.0 ml/min and for the oxygen 3500 ml/min at65 psi. The cooling air was at ambient temperature and the flow rate was25 l/min at 80 psi. The cooling air was directed at the back of thesubstrate with a copper tube whose end was positioned 2 inches from theback of the substrate. The deposition was performed at 700° C. flametemperature which was measured at the substrate surface with a Type-Kthermocouple. The cooling air flow rate can be in a range of 15 to 44l/min. The deposition temperature may vary from 500 to 800° C.

EXAMPLE 12 Zinc Oxide Barrier Layer Deposition

Zinc oxide coatings were deposited onto Cu foil using the CCVD process.During the deposition the solution flow rate, oxygen flow rate andcooling air flow rate were kept constant. The solution of the zinc oxideprecursor contained 2.35 wt % zinc 2-ethylhexanoate, 7.79 wt % toluene,and 89.86 wt % propane. The flow rate for the solution was 3.0 ml/minand for the oxygen 4000 ml/min at 65 psi. The cooling air was at ambienttemperature and the flow rate was 25 l/min at 80 psi. The cooling airwas directed at the back of the substrate with a copper tube whose endwas positioned 2 inches from the back of the substrate. The depositionwas performed at 700° C. flame temperature which was measured at thesubstrate with a Type-K thermocouple. The cooling air flow rate can bein a range of 9 to 25/l min. The deposition temperature may vary from625 to 800° C.

EXAMPLE 13 Tungsten Oxide Barrier Layer Deposition

Tungsten oxide coatings were deposited onto Cu foil using the CCVDprocess. During the deposition the solution flow rate, oxygen flow rateand cooling air flow rate were kept constant. The solution of thetungsten oxide precursor contained 2.06 wt % tungsten hexacarbonyl,26.52 wt % toluene, and 73.28 wt % propane. The flow rate for thesolution was 3.0 ml/min and for the oxygen 3500 ml/min at 65 psi. Nocooling air was used at 350° C. deposition temperature. The temperaturewas measured at the substrate surface with a Type-K thermocouple. Thecooling air flow rate can be introduced in the deposition and directedat the back of the substrate in a range of 7 to 10 l/min. The depositiontemperature may vary from 350 to 800° C.

What is claimed is:
 1. A three layer structure for forming discreteresistors comprising: a metal conductive layer that is patternable intoprinted circuitry by chemical etchants, an intermediate layer formed ofmaterial which is degradable by a chemical etchant, and a layer ofresistive material of sufficient porosity such that said chemicaletchant for said intermediate layer may seep through said resistivematerial and chemically degrade said intermediate layer so that saidresistive material may be ablated from said conductive layer whereversaid intermediate layer is chemically degraded, wherein the layer ofresistive material has a thickness of up to about 50,000 Å.
 2. Thethree-layer structure of claim 1 wherein said intermediate layer acts asa barrier layer to prevent material from said conductive layer fromdiffusing into said resistive material layer.
 3. The three-layerstructure according to claim 1 wherein said intermediate layer is ametal.
 4. The three-layer structure according to claim 1 wherein saidintermediate layer is nickel.
 5. The three layer structure according toclaim 1 wherein the layer of resistive material has a thickness of fromabout 100 to about 50,000 Å.
 6. A three layer structure for formingdiscrete resistors comprising: a metal conductive layer that ispatternable into printed circuitry by chemical etchants, an intermediatelayer formed of material which is degradable by a chemical etchant, anda layer of resistive material of sufficient porosity such that saidchemical etchant for said intermediate layer may seep through saidresistive material and chemically degrade said intermediate layer sothat said resistive material may be ablated from said conductive layerwherever said intermediate layer is chemically degraded, wherein theresistive material comprises platinum.
 7. A three layer structure forforming discrete resistors comprising: a metal conductive layer that ispatternable into printed circuitry by chemical etchants, an intermediatelayer formed of material which is degradable by a chemical etchant, anda layer of resistive material of sufficient porosity such that saidchemical etchant for said intermediate layer may seep through saidresistive material and chemically degrade said intermediate layer sothat said resistive material may be ablated from said conductive layerwherever said intermediate layer is chemically degraded, wherein theresistive material comprises dielectric-doped platinum.
 8. A three layerstructure for forming discrete resistors comprising: a metal conductivelayer that is patternable into printed circuitry by chemical etchants,an intermediate layer formed of material which is degradable by achemical etchant, and a layer of resistive material of sufficientporosity such that said chemical etchant for said intermediate layer mayseep through said resistive material and chemically degrade saidintermediate layer so that said resistive material may be ablated fromsaid conductive layer wherever said intermediate layer is chemicallydegraded, wherein the resistive material comprises nickel.
 9. A threelayer structure for forming discrete resistors comprising: a metalconductive layer that is patternable into printed circuitry by chemicaletchants, an intermediate layer formed of material which is degradableby a chemical etchant, aid a layer of resistive material of sufficientporosity such that said chemical etchant for said intermediate layer mayseep through said resistive material and chemically degrade saidintermediate layer so that said resistive material may be ablated fromsaid conductive layer wherever said intermediate layer is chemicallydegraded, wherein the resistive material comprises dielectric-dopednickel.
 10. A three layer structure for forming discrete resistorscomprising: a metal conductive layer that is patternable into printedcircuitry by chemical etchants, an intermediate layer formed of a metaloxide or a metalloid oxide that is degradable by a chemical etchant, anda layer of resistive material of sufficient porosity such that saidchemical etchant for said intermediate layer may seep through saidresistive material and chemically degrade said intermediate layer sothat said resistive material may be ablated from said conductive layerwherever said intermediate layer is chemically degraded, wherein thelayer of resistive material has a thickness of up to about 50,000 Å. 11.The three-layer structure according to claim 10 wherein the averagethickness of said intermediate layer is a dielectric material betweenabout 15 and about 50 nanometers.
 12. The three layer structureaccording to claim 10 wherein said intermediate layer is silica.
 13. Thethree layer structure according to claim 10 wherein said intermediatelayer is strontium oxide.
 14. The three layer structure according toclaim 10 wherein said intermediate layer is tungsten oxide.
 15. Thethree layer structure according to claim 10 wherein the intermediatelayer is zinc oxide.
 16. A three layer structure for forming discreteresistors comprising: a metal conductive layer that is patternable intoprinted circuitry by chemical etchants, an intermediate layer that isdegradable by a chemical etchant and has an average thickness betweenabout 15 and about 50 nanometers, and a layer of resistive material ofsufficient porosity such that said chemical etchant for saidintermediate layer may seep through said resistive material andchemically degrade said intermediate layer so that said resistivematerial may be ablated from said conductive layer wherever saidintermediate layer is chemically degraded, wherein the layer ofresistive material has a thickness of up to about 50,000 Å.